Mean Well LRS Power Supply

The input voltage goes through the fuse FU1 to the filter, on the elements C1, T1, C2, and C3 and C4.

Next, the rectifier RT1VDM1C5C8R3R4. Rectifier diodes charge high-voltage power capacitors C5, C8, which operate mainly in pulse mode and must pass a large current (10A). At the moment of starting the power supply, a charging current passes through the diodes. RT1 thermistor, which has a high resistance (tens of ohms) in the cold state; When the power supply is turned on, it limits this current, heats up, and its resistance drops.

The rectified voltage is supplied to the half-bridge inverter VT1,VT2,C7,T3. The inverter is assembled according to the self-excitation scheme, for which there is a PIC from the "midpoint" through T2 - there is a special outlet. Circuits VD2, R10, C2, R11, R12, R13 in the bases of power transistors accumulate positive +0.7V to open these transistors. 

The parameters of these circuits are selected in such a way that the inverter without external control is able to generate shortened pulses, which, when rectified, give half voltages (2-3V instead of 5V, and 6-8V instead of 12V), so that an uncontrolled power supply cannot burn the electronic circuits of the computer. An inverter operating in uncontrolled mode can only power the control part of the power supply, and the computer circuits are set to a reset state by the PowerGood signal.

The pulses transformed by T3 are fed to the output rectifier. 

In the +5V/+12V circuits, high-ampere switching diodes VDM2, VDM3 with reduced switching voltage, such as Schottky diodes, are used. To improve performance, the power factor of each rectifier is aligned using R51, C19, R14, C13, R15, C14 chains.

At the output of the rectifier, impulse voltages are obtained with an amplitude about 2 times higher than the nominal one, i.e., for example, at the output of the diode in the +12V circuit, we can see +24V. Since the frequency of the inverter is tens of kilohertz, the smoothing filter is simple and very effective.

Resistors R52, R53, R39, R40 are needed only when the power supply is turned on without load, they create minimal load.

The fan is powered by the +12V output via R38. The need for the R38 is caused by the fact that the fan can fail and short-circuit its supply terminals.

In the control part there is a branch from the +12V rectifier, from the smoothing filter. As mentioned above, at this point the impulse voltage of +24V is doubled. With the help of a diode rectifier VD17, C23, the impulse voltage is converted into almost the same amplitude, but constant. With the R21 and C22 chains, it is also smoothed out. 

In the process of starting the power supply, the inverter creates half voltages at the output of the power supply. 

In particular, on the +12V circuit, the output of the anti-aliasing filter will be 6-8V. At the output of the rectifier TO the filter - 12-14V! It is this voltage that feeds the control circuits. In general, the entire power supply of the control part can be divided into two types: ordinary and stabilized. Normal can vary from +12V to +24V. Stabilization is carried out by the TL494 chip's built-in stabilizer, the output of which is +5V.

First of all, the stable voltage powers the MS TL494. A built-in oscillator is started, the frequency of which is determined by the R31, C28 chain, the sawtooth signal of which is transmitted to the comparators inside the TL494. However, at the moment of start-up, the comparators are "muffled" by the dead time signal applied to the DT pin. This is done in order to "balance" all the transients in the circuit that are present at the time the device is turned on. The R25R30C26 chain is gradually charged and gradually engages more and more of the saw to regulate the voltage.

Output voltage regulation is based on a comparison of the +5V output voltage with the reference voltage. The comparison is organized using two dividers R34R27, R24R28 and a comparator made of TL494. If the output voltage is low, then the TL494 outputs start receiving pulses of additional sway of the inverter. These pulses are applied to transistor switches R20,R32,VT4,VD8,R18,VT9,VD9. The VD11,VD1,2C21 chain creates a voltage of about 1.5V at the emitters of these transistors, which leads to their more reliable closure with a negative (relative to the emitters) voltage with TL494. 

The transistor switches form another VT4VT9T2 inverter, which rocks the main inverter VT1VT2C7T3.

Protection system based on LM339 quad comparator. The purpose of this circuit is to prevent the supply of operating voltages if any one of them is missing or within unacceptable limits. In fact, the circuit can only put the inverter into uncontrolled mode. For example, if there is no +5V, it will not produce +12V/-12V, or if there is no -5V, it should not be +5V.

The task is contradictory, after all, how to turn on such a power supply when there is not a single operating voltage? This is solved by a slight delay, during which the absence of any tension is allowed. Monitoring is organized by the presence of voltages -5V, -12V, by the absence of overvoltage on the +5V line and by excessive oscillation of the control transformer T2 - a clear sign of a power inverter malfunction (it should be self-excited at half power). The +12V voltage is not controlled, because if it is not present, the entire control part of the power supply will not work. The sway level of the T2 transformer is measured by the voltage it induces across the R17R50 resistors. Here they usually put different resistors or mold the solder, apparently they are adjusted at the manufacturer. It is understandable: a transformer, especially a pulse one, is the most difficult element to control.

The voltage from the R17R50VD7 chain is smoothed by the R16C25 filter and applied to the R41R45R46 divider. 

At the same time, +5V from the output of the power supply is supplied to the same divider through VD15R47. 

The reference voltage at the comparators, along the R56R43 chain, is 1.7V. The DA2.2 comparator will be triggered if there is also 1.7V at R45R46. So there should be 5.1V at R47R45. Next, there is a VD15 diode with its 0.7V and finally we get 5.8V – the threshold of overvoltage trip. Since R47 is much smaller than R41, surge protection is always activated regardless of the transformer's sway level. And on the other hand, if there is no overvoltage, it is possible to control the swing of the transformer. It turns out to be a kind of resistive "AND" - independent control of two parameters by a minimum number of elements. 

Monitoring of the presence of -5V and -12V voltages is implemented on the R36R49VD16R48 chain and the DA2.1 comparator. In operating mode, the VD16 diode is always open and the current is always flowing through it to the -12V line. That is, the R48 has a voltage of -5.7V. With the help of the R36R49 divider, this voltage is shifted upwards, but it will still not be enough to trigger the comparator. Now let's imagine that -5V is gone. This is equivalent to having zero potential on the -5V line (thanks to the R53 no-load resistor). At the comparator input at R36R49, the voltage will increase and the comparator will trip. Well, what if -12V disappears? Then the VD16 diode is locked, and the voltage is set to about +5V on the entire divider, so the comparator is activated again.

The signal from both comparators is combined and sent to the delay line implemented on the R44C24R22VT5 chain. The delay generated here is extremely important when starting the power supply. However, if the protection is triggered, two events occur. First of all, the system "clicks" through the VD14. On the R36R49 divider, +5V is permanently wound up, and it will be possible to return the circuit to its previous state only after turning off the power supply and holding it for a few seconds. Secondly, through the VD13, the positive signal discharges the C26 capacitor in the dead-time formation circuit of the TL494. That is, the generator stops generating control impulses, and the inverter is taken into an uncontrolled mode.

The PowerGood signal conditioning circuit starts with the R22C25 chain. Since the time constant of such a chain is about half a second, during this time the power supply will have to be guaranteed to start up and realize that all output voltages are normal. Otherwise, the oscillations will be disrupted and the VT6 discharge transistor will be switched on. This transistor is switched on according to the current circuit, thanks to which it is possible to avoid too high discharge currents C25. The C25 capacitor generates a smoothly changing voltage, which is unsuitable for controlling digital circuits. Therefore, the PSU has a Schmidt trigger implemented on the DA2.3R33R42 chain. The PowerGood output is bound to a +5V output voltage and fed into the computer's motherboard as such.

For the formation of the control voltage and switching of high-power transistors of the UPS converter, a chip TL494CN analogues, IR3M02, uA494, KA7500, MV3759, etc., are used. 

TL594 - analogue of TL494 with improved accuracy of error amplifiers and comparator