Switch Mode Power Supply (SMPS) for Car Amplifiers

This project is very simple car amplifier DC/DC (SMPS) with SG3525 and BUZ11 or you can use any kind of MOSFETs. The output voltage is +/- 35 voltage and it’s regulated and can handle more than 5 Amps.This car amplifier Switch Mode Powder Supply is using a car battery supply from 13V up to 14Volts.

The SG3525A series of pulse width modulator integrated circuits are designed to offer improved performance and lowered external parts count when used in designing all types of switching power supplies. The on-chip + 5.1 V reference is trimmed to ± 1 % and the input common-mode range of the error amplifier includes the reference voltage eliminating external resistors. A sync input to the oscillator allows multiple units to be slaved or a single unit to be synchronized to an external system clock. A single resistor between the CT and the discharge terminals
provide a wide range of dead time ad- justment.
These devices also feature built-in soft-start circuitry with only an external timing capacitor required. A shutdown terminal controls both the soft-start circuity and the output stages, providing instantaneous turn off through the PWM latch with pulsed shutdown, as well as soft-start recycle with longer shutdown commands. These functions are also controlled by an undervoltage lockout which keeps the output off and the soft-start capacitor discharged for sub-normal input voltages. This lockout circuitry include approximately 500 mV of hysteresis for jitterfree operation. Another feature of these PWM circuits is a latch following the comparator. Once a PWM pulses has been terminated for any reason, the outputs will remain off for the duration of the period. The latch is reset with each clock pulse. The
output stages are totem-pole designs capable of sourcing or sinking in excess of 200 mA. The SG3525A output stage features NOR logic, giving a LOW output for an OFF state.

The schematic have the power-on and remote feature. In this design we will use PWM IC with SG3525 from SGS Thompson. The SMPS switching frequency is determined 50Khz. For this, the clock inside IC SG3525 is adjusted 2 x 50 Khz = 100Khz. This clock is built up by pin 5(Ct) and pin 6(Rt). The approach can be done with equation Fclk = 1 /(Rt x Ct). Here we use Ct = 1nF and Rt = 10Kohm.

Pin 1 (Inv In) is the output voltage detector. Pin 1 is connected to the optoisolator type 4N35. Optoisolator is an important component in making this SMPS so we can have  Floating Secondary Ground which will prevent noises (especially whine/storing) if the power amplifier is placed in car. The value of  zener diode is 2 x 35V = 70V.

If it is difficult to have zener-diode voltage of 70 V, then we can series several zener-diodes values until we have total of 70 V. Pin 11 and pin 14 are output pins that will drive the primary winding switching mosfets. Inside IC SG3525 both pins have already opereated in mode push-pull.

Transformator (trafo) for SMPS is self would from ferrite toroidal core (like donuts). It is very important that for SMPS frequency above 20Khz, we cannot use iron core transformator like we use in homes. The ferite core transformator will have black color like in the speaker magnets, but do not have magnetizing force. The basic of equation for switching power supply can be found in KAC-719 Car Subwoofer Amplifier circuit design.

PCB designs are avibale to download in real size on DIYAudio forum.

Source: .http://www.st.com/web/en/resource/technical/document/datasheet/CD00000958.pdf