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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Universal device for testing SMPS. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power Supplies When developing and testing switching power supplies, radio amateurs often encounter a situation where a seemingly correctly assembled power supply "refuses" to work. It is enough to mistakenly change the polarity of at least one of several rectifier diodes at the output of the device or disrupt the phasing of any transformer winding, and the consequences can be the most unpredictable, up to damage to very expensive PWM controllers and switching transistors. A universal tester, which will be discussed in this article, will help prevent such an unpleasant phenomenon. Attention should be paid to the fact that two independent power supplies are used during the SMPS test. One of them, low-current (Imax = 0,2 A), with an output voltage of 10 ... 15 V, after additional stabilization by the DA1 microcircuit at a level of 8 V, feeds the control, indication and protection circuits of the device. The second, high-current (Imax=5A), is a source of test voltage for the tested elements. For this purpose, it is convenient to use a standard SMPS mains rectifier. Therefore, although the transformer T1 and optocoupler U1 in the device provide galvanic isolation between these sources, during the test, in order to avoid electric shock, it should be remembered that the circuit connected to the transistor VT2 and resistor R9 is under mains voltage. If the voltage amplitude of the sawtooth pulses across the resistor R9 exceeds a certain threshold value at which the current emitting diode of the optocoupler U1 will be sufficient to open its phototransistor, the overload signal from the collector of the latter will prohibit the passage of pulses from the generator. A small capacitor C3 connected in parallel to the collector-emitter section of the phototransistor increases the noise immunity of the device. In the described tester, a switching transistor IRFBC40 is used, in which the maximum drain current is 6,2 A, and the drain-source voltage is 600 V. The threshold current level is 5 A, and the protection response voltage will be 0,33 Ohm x 5 A = 1,65, 9 V. The power dissipated by the current sensor (R1) with a duty cycle of pulses D - 1,65 must be at least (2) 0,33 / 8,25 - 0,2 W. When the device is used to evaluate the load capacity of the SMPS (D=8,25), the floor power must be at least 0,2x1,65 = 1,65 W. If the tester is supposed to be used only to test the inductive elements of the SMPS, as in our case, taking into account the sawtooth shape of the current pulses, the power of the resistor should be at least 0,5x0,825 = XNUMX W.
Of course, the imported transistor can be replaced with a domestic KP707V2 or similar, but for them the parameters of the current sensor will need to be recalculated in accordance with the above ratios and taken into account when setting up the device. Consider the work of protection circuits on the elements DD2.1 and DD2.2. The R8C2 circuit is connected to the upper input of the RS flip-flop (pin 3 DD2), the time constant of which is 8,2 ms. It provides a time delay for the appearance of a high level at the input, which is necessary for the trigger of the protection node to reset. This feature is illustrated in Fig. 2 the presence of a time interval tmin between turning on the device and the start of the SMPS test.
In practice, this imposes restrictions on the sequence of switching on the two named independent power sources: first, you should turn on low-current, then high-current, and turn off in the reverse order, first high-current, then low-current. Compliance with this rule will prevent damage to the switching transistor VT2 by the very first pulse at the moment the device is turned on. In addition, I recommend that when you turn on the SMPS for the first time, do not apply the full mains voltage, but gradually increase it, for example, using a laboratory autotransformer. In the event of an overload of the switching transistor, the RS flip-flop switches to the zero state. At terminals 1, 13 of the elements DD1.3 and DD1.4, the high level is replaced by a low one, and further passage of the pulses is blocked. The switched RS flip-flop turns off the HL2 "Check" LED and turns on the HL1 "Overload" LED. The generator on the elements DD2.3 and DD2.4 generates a warning sound signal. After turning off the power and eliminating the overload, after a while, necessary for the discharge of capacitors C1 and C2, the device is ready to be turned on again. The use of a device for estimating the saturation current of the inductor used in the output filter of the SMPS has its own characteristics. Let's consider them in more detail. On fig. 3 shows the connection diagram of the tester in this case.
The power supply unit (PSU) is high-current: its maximum current must exceed the threshold value of 5 A selected for the protection circuits of the device. A diode VD1 is connected in parallel with the choke under test. Here it is permissible to use KD212A or similar. The switching frequency can be very large, especially for chokes with inductances of hundreds or thousands of microhenries. Therefore, for the time of measuring the parameters of the inductor, it may be necessary to significantly reduce the operating frequency with a constant (or adjustable) pulse duration. The performance can also be increased by introducing a VD2 zener diode with an operating voltage slightly higher than the measuring one. It is also desirable that the voltage at the PSU output be adjustable. An oscilloscope is connected in parallel with the resistor R9 of the tester. Possible options A and B of the observed diagrams of the voltage drop on the current sensor Ur9, as well as the voltage U3-and on the gate of the switching transistor are shown in Fig. 4.
As you know, the voltage U applied to the inductor causes a linear increase in the current D1 in it. This dependence is mathematically expressed by the equation AI \u1d (U / L) Δt or, in other words, a voltage of 1 V applied to a choke with an inductance of 1 Gn will cause an increase in current in it by 1 A after 10 s. If the numerator and denominator of the fraction on the right side multiply the equalities by a factor of 6-1, we get an important consequence: to determine the change in current DXNUMX in amperes, the inductance in microhenries can be substituted into the formula, and the time in microseconds, which we will use in measurements. Suppose that the voltage U = 20 V is set at the output of the power supply unit, and with a certain selected inductor, the voltage diagram UR9 takes the form A (Fig. 4). Let us estimate the properties of the throttle. It is obvious that the peak value of the current I = U / R = 0,4 / 0,33 - 1,2 A, and we can conclude that the estimated inductor will be quite efficient when filtering the current up to 1,2 A. Moreover, with using a tester, you can evaluate the inductance of the inductor, for which you need to use the ratio L = (U / AI) At. Substituting the corresponding values, we obtain L = (20/1,2)2 - 33 μH. Of course, many indicators affect the accuracy of the determination: the tolerance of the value of the current-measuring resistor, the error in measuring the voltage and time interval using an oscilloscope, the current-limiting effect in the measuring circuit due to the active resistance of the inductor and resistor R9, and some other factors. But according to the most rough estimates, the total error in measuring the inductance of the inductor by this method will not exceed 20%. Such accuracy is quite sufficient to evaluate the filtering properties of the inductor as part of the SMPS output filter. Now, without changing the inductor, we increase the voltage at the PSU output to 40 V and at the same time we get option B of the diagram shown in Fig. 4. It is important that the peak value of the voltage UR9 does not exceed the threshold level set for the protection circuits, otherwise the measurements will not be possible. As can be seen from the figure, this condition is met. Calculations similar to the previous ones allow us to draw the following conclusions:
A slight discrepancy between the results indicates an increased measurement error, which is associated with difficulties in determining the inflection point on curve B. Usually, a stencil made of paper is used for this, applied to the image of the curve on the oscilloscope screen, as illustrated by line C in Fig. 4. Therefore, during measurements, it is advisable to reduce the voltage at the PSU output to a value at which the diagram takes a strictly linear form, similar to line A, and use the results obtained to evaluate the inductance of the inductor and the saturation current in it. An increase in the probability of saturation in the inductor at a low current is associated with the use of closed magnetic circuits made of a material with high magnetic permeability (more than 200). To avoid saturation, rings made of a magnetodielectric based on alsifer or molybdenum-permalloy alloys should be used, or a non-magnetic gap should be introduced. If we compare ferrite ring, Ш-shaped and armored magnetic circuits, it should be recognized that the last two are more technologically advanced in terms of creating a non-magnetic gap, although it is possible to use ferrite rod segments used in radio receivers for magnetic antennas as weakly saturated magnetic circuits (the lower the magnetic permeability , all the better). And the last option for using the device during testing of the SMPS is as an adjustable load equivalent, and the load is pulsed, which is especially important for power supplies used as part of the UMZCH. Peak, maximum, average, musical, thermal and a number of other terms characterizing the power derived from impulse actions were not in vain invented by specialists to evaluate this class of radio devices. Of course, in this case, the generator in the tester must be rebuilt for the audio frequency range and provision should be made for adjusting the duty cycle of switching pulses, as recommended at the beginning of the article. When measuring, you should pay attention to the thermal conditions of the DA1 chip and the VT1 transistor. It is possible that when the pulse duty cycle is close to 1, it will be necessary to replace them with more powerful elements. Depending on the output power and output voltage of the SMPS, you will need several resistors with a resistance of units or tens of ohms with a power dissipation of 30 ... 50 W. In their absence, it is permissible to use automotive lamps with an operating voltage of 12 V as a load equivalent, and among them it is easy to select specimens designed for a rated current from fractions to tens of amperes. If the maximum power dissipation at a current through the switching transistor of 5 A is not enough for the full load of the SMPS, the high-voltage field-effect transistor IRFBC40 can be replaced with a low-voltage one, for example, IRFZ48N, which has a maximum direct (average) current of 45 A, and a pulse current of up to 210 A. The connection diagram when using the device as an adjustable equivalent of an impulse load is shown in fig. 5.
The ammeter included in the measuring circuit will show the average value of the current. If the ammeter readings are divided by the duty cycle of the pulses, we get the amplitude (peak) value of the current in the load circuit. When the pulse duty cycle is close to 1, the load for the SMPS is maximum. The switching transistor VT2 in the tester should be installed on a heat sink with an area of 100...200 cm2. We will replace the microcircuit stabilizer KR1157EN802A with a foreign analogue 78L82 or more powerful adjustable domestic KR142EN12A, KR142EN12B. It is permissible to replace the K561TL1 chip with the K561LA7. Instead of KT505B, you can use any high-frequency medium power transistor of the corresponding structure. Piezoceramic sound emitter HA1 - any available. Diodes KD522B are replaceable by any low-power silicon ones, for example, KD521, KD522 series, optocoupler - any of the AOT127, AOT128 series. LEDs - any with a clearly visible glow at a current of about 5 mA. Capacitor C1 - any oxide of the specified capacity, the rest - any ceramic. All resistors - MLT, S1-4, S2-23, except for imported R9. Transformer T1 - pulse FIT-5. If this could not be found, the transformer is made independently. Its magnetic core is two K10x6x3 ferrite rings stacked together with a magnetic permeability of 1500 ... 2000. The sharp edges of the rings are rounded with a needle file, the magnetic circuit is covered with insulating varnish and after it dries, 100 turns are wound into two PELSHO 0,12 wires. The transformer should be connected taking into account the phasing of windings I and II, shown in fig. 1. The transformer can also be made on the basis of B14 or B18 armored magnetic circuits. In this case, windings containing 50 ... 70 turns of wire PEV-2 0,12-0,17 should be reliably insulated from each other. Establishing the device begins with checking the parameters of the pulses at the output of the generator (pin 10 DD1). If necessary, they are corrected by selecting the capacitance of the capacitor C4 and the resistance of the resistors R4 and R6. Then, the output of the resistor R10, which is upper according to the scheme, is disconnected and connected to the positive output of the regulated power source, the negative output of which is connected to output 2 of the U1 optocoupler. Gradually increasing the voltage, register the moment of disappearance of pulses at the outputs of the elements DD1.3, DD1.4. By selecting the resistor R10, the absence of pulses is achieved at a voltage of 1,65 ± 0,05 V, after which the connection is restored. At the next stage, by selecting the resistor R5, the current of the LEDs HL1, HL2 is set to about 5 mA. Lastly, check the polarity of the pulses at the gate of the transistor VT2. If they do not match Fig. 2, change the phasing of one of the windings of the transformer T1. The final stage is the control of the operability of the switching transistor VT2, for which the device is connected to the network rectifier of the tested SMPS in accordance with Fig. 5. The SMPS must have a mains voltage switch, a 2 A fuse and an inrush current limiting circuit. As a load, a lighting lamp for a voltage of 220 V with a power of 60 W is used. It is desirable, but not necessary, to include a DC ammeter with a measurement limit of 0,5 A in the circuit. After turning on the mains rectifier, the tester is supplied and removed several times with a supply voltage of 10 ... will show a current of approximately 15 A. Being careful, using an oscilloscope, control the pulses at the drain of the transistor VT0,08. If the transistor is defective, the lamp will glow half as bright as usual and will not respond to turning off the supply voltage to the device. The defective transistor should be replaced, and after an additional check, the device is ready for operation. To expand the capabilities, the device can be supplemented with two switches that switch sets of resistors R4, R6 and capacitor C4 of different ratings, with the help of which several fixed values of frequency and pulse duty cycle are set. Author: S. Kosenko, Voronezh
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