Starter charger with synchronous rectifier. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells

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High-power rectifier diodes generate a significant amount of heat. The reason for this is the unrecoverable voltage drop at the p-n junction of the rectifier diode, reaching 0,5 ... 1 V.

In the proposed device, a synchronous rectifier is used, in which the diodes are replaced by field-effect transistors. The resistance of their drain-source channel in the open state is only a few milliohms. This greatly reduces the voltage drop and hence the heat dissipation.

When using powerful field-effect transistors as controlled valves in a synchronous rectifier, it should be borne in mind that such transistors contain a diode in their structure, connected between the drain and the source in the opposite direction and is usually called protective. Therefore, in the rectifier, field-effect transistors are switched on inversely. While the transistor channel is closed, the current rectifies the protective diode, remaining closed when the polarity of the applied voltage is reversed for it and opening when it is direct. To eliminate the voltage drop across an open diode, it is necessary to open the transistor channel synchronously with it, applying opening pulses to the gate. As a result, almost all the current will flow through the channel, the resistance and voltage drop on which is much less than that of an open diode.

For successful operation, a synchronous rectifier must contain a device that monitors the polarity of the voltage applied to the valves - field-effect transistors and generates control signals that open and close them in a timely manner. This is especially important when operating on a capacitive load or on a load that has its own EMF (battery).

Fig. 1

On fig. 1 shows a diagram of a charging device based on a synchronous rectifier. It works as follows. Let a positive voltage half-wave act on the drains of parallel-connected transistors VT2 and VT4. The protective diodes of the transistors are closed with this voltage polarity.

Limited by the diode VD2 to +0,7 V, this voltage will go to the inverting input of the comparator DA1. As a result, at the inverse "emitter" output of the comparator (pin 1), the voltage level will be high. Since the Schmitt trigger, built on the DA3 timer chip, inverts this level, the voltage between the gates and sources of transistors VT2 and VT4 is close to zero, and the transistors themselves are closed.

During a negative half-wave of voltage between the drains and sources of transistors VT2 and VT4, the protective diodes of these transistors open. But since the voltage at the inverting input of the comparator DA1 is now less than at the non-inverting one, the level at its output 1 will become low, and at the output of the DA3 chip it will be high. The drain-source channels of transistors VT2 and VT3 will open by shunting the protective diodes, and will remain in this state until the polarity of the voltage applied to them changes.

Similarly, the control of transistors VT3 and VT5 of the second arm of a full-wave rectifier takes place. The use of KR1006VI1 timers with an output current of up to 200 mA as drivers ensures fast switching of VT2-VT5 transistors, which further reduces the power dissipated by them.

Capacitors C1, C2 eliminate the high-frequency ripple voltage supplied to the inputs of the comparators DA1 and DA2, ensuring their switching without "bounce". Resistors R8 and R9 are load resistors in the emitter circuits of the output transistors of the comparators. The collectors of these transistors are connected to the power plus. When it is turned on, the R10C4 circuit sets the timers DA3 and DA4 to their initial state with a low level at the outputs. As the capacitor C4 charges, the voltage across it increases and the microcircuits begin to work normally.

The use of two transistors in each leg of the rectifier allows you to bring the load current up to 200 A, of course, with a sufficiently powerful transformer T1. This is quite enough to "help" the starter of almost any car to start the engine if the battery is not sufficiently charged. The voltage on the windings II and III of the transformer under load should be about 10 V, and the overall power should be at least 800 V-A.

For charging, the battery is connected to the "U ^ p" and "Common" terminals. The DA5 comparator compares its voltage with the reference one. The comparison threshold is set by the tuning resistor R4. While the battery voltage is below the specified level, the level at the output 7 of the comparator DA5 remains low, and the transistor VT1 is open. The synchronous rectifier charges the battery.
Since the voltage at the output of the synchronous rectifier is pulsating, as the battery is charged, it exceeds the exemplary one for most of the half-cycle. The voltage level at the output of the comparator in these time intervals becomes high, and the transistor VT1 closes. Due to this, the average charging current decreases, and when the battery is fully charged, it becomes equal to zero (transistor VT1 is constantly closed).

The synchronous rectifier control unit is powered by a rectified voltage through the diode VD1. Capacitors C5 and C6 smooth out ripples.

Fig. 2

A drawing of the printed circuit board of the control unit is shown in fig. 2, and its appearance is shown in Fig. 3. Instead of capacitors C5 and C6, one capacitor with a capacity of 4700 microfarads is mounted here. The module of field-effect transistors VT1-VT5 fixed on heat sinks is shown in fig. four.

If the current consumed from the synchronous rectifier certainly does not exceed 100 A, one transistor can be left in each of its arms. And if a current of more than 200 A is needed, the number of transistors connected in parallel in each arm can be increased accordingly or replaced with more powerful ones, including IGBTs. For example, IGBT GA400GD25S are rated for 400 A, GA600GD25S - 600 A.

Author: V. Kalashnik, V. Chernikov, Voronezh; Publication: radioradar.net

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