Theory of operation for the Mighty-Mite 700 5V power supply. Disclaimer: This was reverse engineered so don't expect more than 90% accuracy on this. The power supply has two outputs: V1 is 5V, 60A and V2 is 5V, 12A. I'll call these the "big supply" and the "small supply". Maximum wattage in total is 300W. Internally, the power supply has a half-bridge topology. It uses two switching transistors along with three large capacitors that create a "neutral". The transistors are switched on alternately at fixed frequency; reducing the transistor on time (PWM) controls the output. A half-bridge power supply is more complex than a flyback or forward converter, but provides high output power. It also reduces voltage stress on the switching transistors, since each one only sees half the input voltage. Primary side The AC power goes through a transformer that converts 120/240V to 120V. This transformer is connected as an autotransformer for some reason; its primary and secondary are connected together. The 120/240V change is done through jumper plates that cause the primary coils to be wired in parallel (120V) or series (240V). The fan is powered from the 120V output. This transformer has an additional winding that powers the control circuitry. From here, the 120V AC goes to a daughter board that generates +/- 170V DC to drive the transformers. It contains a bridge rectifier and two very large capacitors. Note that the capacitors are in a voltage doubler configuration, connected to the AC neutral. The other large capacitor drives the other side of the transformers. This is a typical half-bridge circuit. The remaining resistors and capacitors form a snubber to reduce switching transients. The +/- 170V DC outputs are fused, since if the transistors short out, it would cause a 340V short circuit. The wiring is as follows: yellow: 120V AC in green: AC neutral orange: other AC neutral, different for 240V input brown: +170V to switching transistor blue: 0V to transformers red: -170V to switching transistor gray: snubber: connected to transistor outputs purple: snubber: connected to to transformers The switching transistors are 13SE150 NPN power transistors, which are not well documented. The transistors switch out of phase to drive the transformer alternately with plus and minus 170 volts. (Note that this half-bridge topology is different from a flyback power supply, for instance.) The transistors are driven at a constant frequency; PWM is used to regulate the supply. The switched output from the transistors goes to two transformers in parallel; one for the big supply and one for the little supply. (Optionally, a third transformer can be installed for a second little supply.) The transistor output also goes through a toroidal coil which apparently measures the drive current for the current limiting circuit. Secondary of the big supply The big supply's secondary is straighforward. The transformer's center-tapped secondary winding is rectified by two large diodes (on the heat sink). (A resistor-capacitor snubber is connected across the diodes). The output is filtered by a large choke (which looks like the transformers), and two large output capacitors. Two ferrite rings between the output capacitors provide additional filtering. Control of the big supply The big supply has a large amount of control circuitry to provide the switching power supply regulation. The transistors are switched alternately at 23 kHz, with the duty cycle modified to provide regulation. The switching clock is generated by unijunction transistor Q5 (2N4871), buffered by Q6. The clock is 45.8 kHz: pulses low for 2.8 microseconds and high for 19 microseconds. Since the transistors are driven alternately, the overall switching frequency is half of this, 23.9 kHz. The 74110 flip flow does this divide-by-two. The sense inputs provide the voltage feedback for regulation. The sense voltage and the reference voltage go into a comparator (a differential pair formed by Q7 and Q8) to create the feedback signal. The reference voltage comes from Zener diode CR8, resistors 19 and 21, and diodes 9 and 10, and is adjusted by the variable resistor on the panel. (It appears that regular diodes are just labeled with a number while Zener diodes are labeled with CRn.) The role of diodes 9 and 10 is unclear; maybe stabilization. The feedback signal goes through Q14 to the 74112 multivibrator chip to provide the PWM control. This chip generates a variable-width "blanking" signal, that extends the transistors' off time beyond the minimum 50%. The multivibrator is triggered by the clock and the pulse width is controlled by the feedback signal. Control is slightly unusual, wired into the normal R-C timing. The multivibrator time constant is controlled by R46 (3.3K) and C18 (3.3nF), yielding a time constant of 3.8 microseconds. However, Q14 is connected to pin 11 (via R43), modifying this time constant through a "trick". Current drain through R43 will cause slower charging of the timing capacitor, extending the pulse. The clocks signal from the flip flop and the blanking signal from the multivibrator are combined by the 7437 quad NAND gate to form two transistor drive signals and two complemented drive signals. One complication with a half-bridge power supply is one switching transistor is 170 volts above the other, so the base drive voltages are not straighforward to generate. This power supply uses base drive transformers (T4 and T5) to generate the base voltages. Each transformer has three windings: an "on" winding, an "off" winding, and the output winding. These windings are driven by transistors Q16-Q19, driven by the NAND gate outputs. 18 volts fed into R53 and R54 powers the windings. A bunch of resistors, capacitors, and diodes are part of this drive circuitry. The control circuitry is powered by 18 volts. This is produced from a winding on the input transformer, a bridge rectifier and filter capacitor, and a 7418 voltage regulator. For the TTL circuitry, power resistor R20 and Zener diode CR7 provide 5 volts. An over-current sense detects excessive current to the transformers. It consists of a toroidal sense transformer between the switching transistors and the transformers. The sense transformer's output is rectified by diodes 19-22. Variable resistor R65 sets the level, along with a confusing circuit with R60, R63, R64, C29, C30, Q20 and diode 18. The output goes through Q13, Q12, and Q10 before shutting down the supply by pulling the base of Q11 low. The power supply also includes over-voltage protection. Variable resistor R32 sets a level between the positive and negative output voltages and feeds Q9. An over-voltage condition triggers SCR1, which pulls the base of Q11 low, shutting down the supply. A thermal switch (TS1) on the switching transistor heat sink will switch off the input voltage to the 7818 regulator. This will shut down all the control logic if overheating occurs. Note that the control circuitry is isolated from AC; it is powered by a separate winding, and the switching transistors are isolated by the drive transformers. Thus, no optoisolators are required for the voltage feedback signal (unlike many power supplies). Secondary and control of the little supply The little supply's secondary is more complex. The big supply is regulated directly by the switching circuitry. The little supply's transformer "piggybacks" off the big supply, so it is not directly regulated. Instead, the little supply has a linear regulation stage. The little supply has a center-tapped transformer secondary that is rectified by a heat-sinked dual diode that resembles a power transistor. The DC is filtered by an inductor and capacitor. A 2n5886 power transistor and a power resistor drop the voltage to 5 volts. The power transistor is controlled by a 723 voltage regulator IC. The 5V output has a protection diode and a couple filter capacitors. The output is also protected by a crowbar SCR. This regulation circuitry is powered by a second winding on the transformer, bridge rectifier diodes, and a filter capacitor. Unused circuitry The power supply has unpopulated circuitry on the PCB for a second small supply, essentially duplicating the first small supply. There are four unused terminals to support this supply. Part of the PCB is also unpopulated for the big supply's control circuitry, conneted to external terminals 5 and 6. It's unclear what this circuitry would support. Based on other LH power supplies, I suspect an external ON/OFF control and a PF (power fail) indicator. Terminal 5 appears to be a power-good indicator. Diodes 3 and 4 rectify the AC from the control-supply winding (the same winding that generates the 18V for the control section). The resulting DC voltage indicates the level of the AC supply. Variable resistor 2 divides this control voltage and feeds it through Zener diode CR5, setting the power-good threshold. Transistors Q1, Q2, Q3 (PNP) and Q4 convert this into a sharp TTL output. The key idea behind this circuit is the rectified sense voltage has a smaller filter capacitor than the 18V supply does, so if there is a power failure the sense voltage will drop before the control voltage does. Terminal 6 appears to be an external on-off control. It is simply connected through a resistor to the base of transistor Q3. This will shutdown the power supply through the same path as an overcurrent shutdown. I hope they didn't charge much for the external on/off feature since it's literally a single resistor.