SMPS Switched mode power supply, 12 V to +/- 40 V

Switched Mode Power Supply SMPS by SM5UIU - In +13,5 VDC / Out +/- 40 VDC

I have been interested in audio amplifier's for some time now and one thing they realy need is a power supply. This is not normaly a problem as long as you have access to 110/220 VAC but if you would like to use the amplifier in eg a car this surely becomes a problem. So we have to build ourselves a converter that convert's the battery power +13.5V (when charging) to eg. +/- 40 VDC or whatever voltage we need.

The input filter, filter's out all the voltage spikes from the SMPS primary side, and leaves the +13.5VDC side clean an ripple free. Voltage is applied to Vin and C11 / C6 along with C7 / C12 help's to deliver current to the SMPS when needed. L1, which is a copper wire dia 2.5 mm wound on a ferrite corre, stop's all possible voltage spikes from distorting the battery +13.5V line.

The regulator circuit takes care of all the switching action. Compares the final output voltage, drives the transistors with PWM technique and is the heart of the whole SMPS. The switch regulator that I have chosen is the SG2524 from Thompson electronics. There are "lot's" of other regulators on the market and by studying their datasheets and examples will give you some god hint's on how to make your own SMPS.

First we apply the input voltage to Vin and Colector A / B (the IC has two transistors intregrated which can be configured in many different way's, here we tie their collectors directly to +13.5V). Their emitters go via R5 & R6 to base of Q7 & Q8 which are used as emitterfollower's. The emitter's of Q7 & Q8 drive the output transistors and R9 & R10 serve as emitter load for Q7 & Q8 to quickly discarge any capacitive charges that could delay the ON/OFF transition.

VREF is at +5.1 V and is divided by R1 & R4 which in turn suplies the NONINVERTING input of the error amplifier with VREF/2 Volt (why VREF/2, well all datasheets seem to like this approach so why not). The VSENSE terminal is the actual output voltage, in our case +40 V. We asume that the audio amp will use the same current from both the positive and negative voltage rail so we only need to "sense" the positive voltage. When we have +40V on VSENSE the voltage drop across R7 will be equal to the voltage over R4. R3 and C1 set's the operating frequency (>100kHz). C3 & R2 are connected to the compensation input.

From Q7 & Q8 emitters the switching signal goes to DRIVE 1 & 2 respectively which drive the output switching type MOS transistors, 3 in parallell on each side (due to the high current). Each transistor has a 100 ohm resistor to their gate to separate them from eachother and protect Q7 & Q8 in case of a short in one of the output transistors. The primary winding consists of 2 separate windings with 4 turn's each. Each winding is connected to Q1, Q2, Q3 and Q4, Q5, Q6 respectively. When DRIVE1 is "HI", DRIVE2 is "LO" and vise versa. When eg DRIVE1 goes from "HI" to "LO" there is a short "deadtime" before DRIVE2 may go "HI" to ensure that the transistors have changed to an OFF state. In my design I have only used "one" solid wire on each winding but as the frequency increases the current moves further out on the condutor and that's why we would consider using perhaps 2 or more thinner wires wound together as one, this to help keep losses as lo as possible and also that in turn will keep core temperature down. The secondary winding consists of 2 windings with 18 turn's each. The turn ratio is very dependent on what in/ouput voltage you have and the power transformed.

R19 / C9 and R18 / C10, filter out some unvanted "signal's" when the transistor's go to OFF state. Component's value depend on frequency and the current that the primary side has been designed to handle, which in turn desides how large the voltage spikes will be.

A word of caution when handeling high power levels. If you would have poor ground and/or power cables with a small area to the SMPS unit itself, will affect system performance negatively. As the input voltage decreases due to cable losses the current through the output transistors will increase to a point of destruction. So if you design a SMPS for +13.5V in then make every effort to keep the voltage at that level. Also the effiency will vary depending on input voltage and output load.

You could also consider to put in a voltage monitoring circuit which will monitor the input voltage and disable the SMPS when the voltage get's to HI or LO. Also a current sensing device could be implemented, for protection.

The ferrite ring core used for the voltage transformation should be specially designed for use with the choosen switching frequency and of proper dimension. Even if push/pull style switching suplies do have advantages when it comes to keep the core from not saturating it's better to use a larger core if you aren't sure of the core capabilities. Again, check the datasheet's.

As the frequency increase there is not only a problem with capacitors, transistors and so on. We will also need special switching type diodes capable of handeling the current. "Normal" diodes are to slow for this kind of high frequency rectifing. Diodes used here is BYW29 with a TO220 case rated for 150V / 8 Amp average current - 16 Amp repetive peak current. Diodes D1-4 are ofcourse mounted on the heat sink.

R17 is used as an "ground" isolator between the in/output voltage ground and it's main function is to provide the SENSE input a ground reference. Some comersial SMPS manufacturers have choosen to use an optocoupler in conjunction with a zener/resistor network as the sensing elemenent and feedback to the regulator.