Application of medium power solid-state optoelectronic relays. Reference data

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The article introduces some features of medium power optoelectronic relays produced by CJSC "Proton-Impulse". The information given in it will be useful to all readers who use or develop various thyristor and transistor power circuit switches.

The table gives an idea of ​​the designation system and the range of produced relays.

More detailed information about them can be found on the manufacturer's website. . All optoelectronic relays can be divided into two main groups: AC with power elements on triacs and trinistors, unipolar and bipolar DC with IGBTs or MOSFETs in power circuits. Their fundamental difference is that AC relays are characterized by partial controllability - the power circuit always breaks only at zero current. This creates certain advantages for inductive loads, eliminating the overvoltage impulses that occur when switching off. It is very difficult to use such relays in DC circuits. But bipolar DC relays are capable of switching alternating current.

One of the criteria for choosing a relay for a particular application can be the power dissipated in its power element. When operating in AC circuits with a voltage of 220 ... 380 V and currents of more than a few amperes, thyristors are 3 ... 5 times better than IGBTs in this indicator. The ratio of power dissipated by IGBTs and MOSFETs is approximately equal to the numerical value of the current in amperes.

AC RELAY

Among thyristor relays there are single-phase normally closed and normally open relays for current 1...100A; three-phase normally open for current 10 ... 100 A; one-, two- and three-phase reversible for current 10...40 A with built-in protection against phase-to-phase short circuit and instant reverse; dual for a current of 1 A or more with independent control, with and without a common point at the output.

The relay class according to the breakdown voltage of the output can be from the fourth (at least 400 V) to the twelfth (at least 1200 V), and the permissible peak value of the insulation voltage between the input and output current-carrying circuits and the heat sink is 1500 or 4000 V.

The relay with the TM index provides control of the zero phase of the switched voltage (they turn on only when the instantaneous value of this voltage is close to zero, which reduces the generated interference). Relays with the TC index do not have this property.

Relay control circuits are current (Fig. 1, a, rated current - 10 ... 25 mA) or potential (Fig. 1, b - constant voltage 4 ... 7 or 3 ... 30 V, Fig. 1, c - variable 6..30 or 110...280 V). With current control, only single-phase and two-channel relays are produced, with potential - of all types. In various modifications, the place of the resistor R1 (see Fig. 1,6 and c) can be occupied by a current stabilizer, and the "quenching" capacitor C1 (see Fig. 1, c) can be absent. If the relay (for example, polyphase) has several emitting diodes, they can be connected in series or in parallel.

Thyristor structures are very sensitive to exceeding the allowable voltage, which leads to irreversible breakdowns. The main technique for protecting the relay output is shunting it with a varistor. Varistors CH2-1, CH2-2 are recommended with a non-linearity coefficient of more than 30 and a dissipation energy of 10...114J. When choosing, one should proceed from the fact that the classification voltage of the varistor (at which the current through it reaches 1 mA) must exceed the amplitude value of the switched one and be lower than the breakdown voltage of the thyristors. It is necessary to take into account the possible instability and technological variation of these parameters. Ceteris paribus, switching higher current requires relays of a higher voltage class. This is due to the dependence of the voltage on the outflow varistor.

Another feature of thyristor structures is sensitivity to the rate of voltage rise (dU/dt) applied to a closed device. Exceeding the critical speed leads to its unauthorized opening. Large values ​​of dU/dt are possible when voltage is applied to the load circuit at a moment close to the maximum of the sinusoid. They can be caused by impulse noise in the switched circuit or voltage surges when the load circuit is broken of an inductive nature.

To reduce dU / dt and prevent undesirable consequences, the outputs of thyristor relays are shunted with damping RC circuits, the element values ​​of which are selected experimentally. Usually they lie in the range of 20 ... 50 ohms and 0,01 ... 0,1 μF.

An additional means of increasing the resistance of the relay to voltage surges is a delay reactor connected in series with the load. It is an inductor wound on a magnetic core with high magnetic permeability and a rectangular hysteresis loop. At operating currents, the magnetic circuit is saturated, the inductance of the reactor is small and it does not affect the ongoing processes. The inductance that grows with decreasing current slows down its change and delays the voltage reversal, helping to close the thyristor.

By reducing the rate of current rise at the initial stage of turning on the thyristor, the reactor contributes to a more uniform distribution of current over the cross section of the semiconductor crystal, which prevents local overheating. This is especially important when the relay with the TC index operates on a capacitive or active load or in the phase-pulse power control mode. In addition, the reactor, by increasing the impedance of the load circuit, increases the efficiency of the varistor protection.

For thyristors operating on an inductive load, there is a danger of overcurrent due to the asymmetry of the moments of switching on in the positive and negative half-cycles, leading to the appearance of a constant component of the flowing current, saturation of the load magnetic circuits, and, consequently, to overcurrents.

Current overload can also be associated with saturation of the magnetic circuits of inductive loads (idling transformers, control windings of contactors) if the direction of their residual and current generated at the moment of magnetization is turned on.

The starting current caused by this can be ten times higher than the nominal one, and the case of switching on at the moment the voltage phase passes through zero is the worst. It is optimal to turn on the thyristor at the maximum voltage or "softly" start it, starting from small conduction angles. To work on an inductive load, it is recommended to use relays with the TSI index, designed for increased surge current.

The asymmetry of the turn-on moments can be a consequence of the difference in the turn-on voltage of thyristors in different polarities. It plays a significant role if the amplitude of the switched voltage slightly exceeds the turn-on voltage of the thyristor (5 ... 15 V). Asymmetry also occurs with incorrect phase-to-pulse control of the relay, as well as when the thyristor is opened not in every half-cycle due to the fact that the reverse voltage crosses the turn-on "window" too quickly. The last factor is one of the main ones that limits the frequency of the switched voltage (usually no more than 500 Hz).

Operation on a capacitive load is characterized by the possibility of large current surges in the power circuit and the effect on the thyristor of a voltage that reaches twice the amplitude of the switched one.

An inrush current occurs if the relay is switched on with a non-zero phase of the switched voltage. Connecting a discharged capacitor with a capacity of 220 μF to an alternating current network of 50 V 100 Hz can cause an inrush current with an amplitude of up to 31000 A. The rate of current rise in a load with an inductance of 1 μH reaches 310 A / μs at a maximum allowable value for thyristors of 20 ... 160 A / ms.

Since the turn-on voltage of the thyristor is different from zero (as noted above - 5 ... 15 V), current surges occur in each half-cycle of the switched voltage. With a load capacitance of 100 microfarads, the amplitude of such surges is 500 ... 1500 A. They generate significant electromagnetic interference and powerful high-frequency components in the load current spectrum. The latter are very dangerous for some capacitors, causing them to overheat and breakdown. Therefore, to work on capacitive loads, it is necessary to use a relay with control of the voltage phase transition through zero and with a low turn-on voltage, for example, with the TMK index, for which the turn-on (4 V) and turn-off (10 V) voltages are normalized.

It is known that after the current drops to zero and the thyristor is turned off, the load capacitance remains charged to a voltage close to the amplitude of the switched one. In the next half-cycle, the sum of this voltage and the mains opposite polarity will be applied to the closed thyristor, which can reach a double amplitude, for example, at a mains voltage of 380 V ± 10% - 1170 V. Under these conditions, a relay of even the highest, twelfth voltage class will work at the limit of its capabilities and it cannot be protected from breakdown by a varistor.

In such cases, it is advisable to use relays not only switched on, but also switched off at zero voltage, for example, bipolar direct current. This eliminates voltage overloads, significantly expands the operating frequency range, but somewhat worsens energy performance. For operation at frequencies up to 1 kHz, samples of the 5P 66 series relays have been developed, and work is underway to expand their frequency range to tens of kilohertz.

On fig. 2 shows a diagram of using a single-phase reversing relay U1 to change the direction of rotation of a single-phase electric motor M1 with a phase-shifting capacitor C1.

On fig. 3 shows a diagram of a two-phase relay for controlling a three-phase motor. The switching elements of the relay are conventionally depicted as triacs, although in some cases these are trinistors connected in anti-parallel.

Relay control circuits are not shown in the diagrams. They must be arranged in such a way as to exclude the simultaneous supply of signals to open triacs VS1 and VS2 (see Fig. 2) or VS1 and VS4, VS2 and VS3 (see Fig. 3). Only one of each pair should be open at any time. However, due to the triacs turning off only at zero current, after a reverse signal is given, some of them may still be open at the same time. In a single-phase device, this will lead to the discharge of the phase-shifting capacitor C1 through triacs, in a three-phase device, to an interphase circuit.

To avoid such situations, reversing relays have a hardware turn-on delay of 20 ... 30 ms, due to which, at a network frequency of more than 40 Hz and an "instantaneous" reverse, open triacs have time to close.

There are other reasons why thyristors sometimes turn on at the same time. For example, the slew rate of voltage supplied by an electromagnetic starter may be higher than the critical one for two devices connected in series. Damping RC circuits are of little help in this case, as they are shunted by the extremely low mains impedance. Large dU/dt values ​​can be caused by transients or switching surges.

Provided in the device according to the scheme shown in Fig. 3, inductors L1, L2, in interaction with capacitors C1-C4, reduce the rate of voltage rise, reducing the likelihood of phase-to-phase short circuit. In addition, their inductance limits the rate of current rise, large values ​​of which are destructive for thyristors.

However, neither snubber circuits nor inductors guarantee the impossibility of phase-to-phase faults. The generally accepted method of protecting thyristors from their consequences (it is recommended for their products, for example, by Motorola, Siemens, Opto-22) is the installation of current-limiting resistors R1 (see Fig. 2) and R1, R2 (see Fig. 3). Their ratings are chosen so that the phase-to-phase fault current does not exceed the allowable for the surge current relay used. The duration of its flow does not exceed half the period of the mains voltage. The consequences of installing limiting resistors - a decrease in voltage on the motor windings and the need to remove the generated heat - have to be put up with.

DC RELAY

DC relays with output circuits based on IGBTs and MOSFETs are available in single and double poles. In the last two output transistors are connected back-to-back. For MOS transistors, this is necessary so that the closed channel of one of them prevents the flow of current through the forward-biased shunt diode of the second (such diodes are necessarily present in the MOS structure). Diodes have to be introduced into IGBT structures on purpose, but already to pass the current flowing in the opposite direction for the transistor.

Note that the so-called multi-channel DC relays with various combinations of normally-closed and normally-open output circuits are also produced. When applying them, it should be taken into account that the output circuits become normally closed only after the supply voltage is supplied to the relay from a source galvanically connected to the control inputs.

The residual voltage at the output of unipolar relays on MOS transistors in the open state depends on the channel resistance of the latter at a temperature of 25 ° C, which ranges from a few milliohms for low-voltage transistors to a few ohms for high-voltage transistors. With an increase in the temperature of the crystal to the limiting one (150 °C), this resistance increases approximately twice.

Bipolar relays on MOSFETs have a higher residual voltage. It consists of voltage drops across the channel resistance of one transistor and across a forward-biased diode shunted by the channel resistance of the second transistor. The current-voltage characteristic of the output circuit of such relays in the on state at low current is almost linear, then gradually turns into a diode characteristic. The inflection point lies in the region of 100 ... 200 A for low-voltage relays and units of amperes - for high-voltage ones.

The control elements of the output transistors in the relays of the 5P 20 (unipolar) and 5P 19 (bipolar) series are photovoltaic optocouplers with an output current of the order of a few microamperes. For this reason, the charging of the gate-source capacitance of MOSFETs is quite slow, which leads to a relay turn-on delay of tens of milliseconds. The turn-off delay is much less (no more than 1 ms), since special thyristor discharge units of the mentioned capacity are provided.

High-speed relays are characterized by turn-on/turn-off delays of a few microseconds, but they require an additional power supply for the control circuits. For relays of various types, this source must be galvanically connected to the output or input of the relay.

Relays powered by the input of the 5P 57 (bipolar) and 5P 59 (unipolar) series, with on / off delays of a few microseconds, are capable of switching at a frequency of no higher than 10 ... 20 Hz, since the photovoltaic optocouplers used in them cannot replenish quickly enough energy dissipated during shutdown.

Unipolar output-powered relays of the 5P 40 series can operate at a switching frequency of tens of kHz. For their power supply, a voltage source of 10 ... 15 V isolated from the input circuits is required. 
Relays with input power supply of the 5P 62 series also have similar parameters, however, several external elements must be connected to them, the ratings of which are selected based on specific conditions.
In principle, both IGBTs and MOSFETs are capable of withstanding an avalanche without failure. However, the permissible breakdown energy is small (tens to hundreds of millijoules) and the probability of failure is quite real. This implies the need for protection against voltage surges in the power circuit. For bipolar relays that switch AC circuits, all that has been said above about the protection of thyristor relays is true. Unipolar relays can be protected by a zener diode or a varistor shunting the output.

A common way to protect against high voltages that occur when an inductive load is turned off is by shunting it with a diode in reverse polarity. The current I, flowing through the load before the circuit breaks, in this case decreases exponentially with the time constant L / r, where L and r are, respectively, the inductance and resistance of the load. Part of the energy

the load stored in the inductance is dissipated by its active resistance, the other - by the shunt diode.

It can be shown that for small values ​​of r, the bulk of the dissipated energy falls on the diode. This causes an overload of the latter in terms of pulse, and at high switching frequencies - in terms of average power dissipation.

If the maximum allowable voltage of the transistor Udop is significantly higher than the switched Ucom, the operating mode of the protective diode will greatly facilitate the inclusion of a resistor with a nominal value in series with it

In this case, at the moment of switching off, the voltage at the output of the relay is equal to other + RI, energy is released on the diode

(where Ud - 0,7 V is the direct voltage drop across the diode), and on the resistor -

Therefore, at the switching frequency fkom, the power of the resistor must be at least

The introduction of a resistor has another positive effect - it reduces the load off time, since the time constant of the current decay in this case is equal to L / (R + r).

Relays of the 5P 19, 5P 20 series, as already noted, are characterized by a turn-on delay of tens of milliseconds, which limits the maximum frequency

where lK0M is the switched current. Since the duration of the current decay during switching off is an order of magnitude less than tout, the energy dissipated in this case can be neglected.

Potentially dangerous for the power transistors of the relay are two modes of operation: switching a stationary load with a frequency close to the limit, and switching on a load with a large starting current (for example, the starting current of an incandescent lamp is more than 10 times higher than the nominal one).
In the first case, the average power dissipated by the relay is equal to

where ROTKr is the resistance of the output circuit in the open state; Q - duty cycle (the ratio of the switching period to the duration of the on state). For example, on a unipolar relay 5P 20.10 P-5-0,6 (limiting voltage - 60 V, current - 5 A, R - 0,055 Ohm, thermal resistance of the crystal-environment - 40 ° C / W) at a load current of 5 A in in a permanently on state, a power of no more than 1,375 W will be released, which will cause an acceptable in most cases overheating of the crystal relative to the medium by 55 °C. However, switching the same load with a frequency of 10 Hz with a duty cycle of 2, a voltage of 50 V and tout = 5 ms will lead to an increase in the released power up to 2,77 W and overheating of the crystal by 110 °C. This will not allow the relay to operate reliably at an ambient temperature above 40 °C.

In the second case, the initial value of the load current is much higher than the nominal value, so the turn-on energy of WBKJ1 may exceed the allowable value for the relay transistors. Since with a decrease in t, the switching energy decreases proportionally, it is advisable to switch inertial loads using high-speed relays, for example, series 5P 57, 5P 59.

As noted above, relays of the 5P 62 series for operation at a switching frequency of more than 10 ... 30 Hz require the connection of additional external elements. Like the relays of the 5P 57 and 5P 59 series, their internal voltage source for the control circuit of the output transistor is of low average power and cannot quickly replenish the energy expended when the transistor gate capacitance is discharged. To eliminate this shortcoming, an external capacitor is designed, through which, when the output transistor is turned off, additional energy is "pumped" into the control circuit from the source of the switched voltage. The optimal capacitance of the capacitor depends on the operating conditions of the relay, in particular, on the switching voltage. Therefore, it cannot be introduced into the relay.

Each time the input transistor turns on, the capacitor is discharged through the gate drive circuit, dissipating C U²/2. If the switching frequency is high enough, the additional power dissipated in the relay reaches an unacceptable value. To reduce it, a resistor is used, on which a significant part of the energy stored by the capacitor is dissipated, and a zener diode. The stabilization voltage of the latter is chosen so that at the minimum value of the switched voltage, the capacitor is charged only up to 15 V.

THERMAL CONDITION OF THE RELAY

For relays operated without a heat sink, the maximum switched current is normalized based on the limiting temperature of the power element crystals Tcr. max (125 °C - for thyristors, 150 °C - for transistors) at ambient temperature Tacr = 25 °C. The same parameter of a relay with a heat sink is set according to the limiting temperature of the crystal at a heat sink temperature Tto = 75 °C for thyristor relays and Tto = 90 °C for transistor ones. The last two values ​​are chosen from a rather arbitrary condition of equality of the thermal resistance of the external heat sink RT0 to the "equivalent" thermal resistance of the crystal-heat sink R3kb- It should be borne in mind that in the reference data of polyphase relays, the thermal resistance is usually indicated on the basis of "per phase", therefore "equivalent" the resistance, for example, of a three-phase relay is three times less.

The main relation for thermal calculations: Tacr + P(RTO + Ieq) < Tcrlop, where P is the power dissipated by the relay. An example of how to calculate this power for a DC relay with a MOSFET output was given in the previous section. For IGBT, it is calculated by the formula P = UOCT-lKOM, where UOCT is the residual voltage on the open transistor. The power dissipated in one phase of the thyristor relay is calculated by the empirical formula P = (0,145 + 0,7UOCT peak) Ieff, where U0CT peak is the peak value of the residual voltage on the included thyristor; Ieff is the effective value of the current flowing through it.

Author: S. Arkhipov, Orel

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