Switchmode Boost Power Supply


More photos and oscilloscope shots to follow.


A quite unique switching power supply with two 555 ics, one Astable asymmetrical and pulsing the half rail voltage pin 2 trigger on the second 555 which runs in monostable mode.  The negative narrow pulse from IC1 triggers the start of IC2 charge cycle and that pulse voltage is changing as V supply changes, so ensuring that the frequency of operation and therefore pulse width in IC2 remain constant over a wide supply voltage range.  Control voltage for IC2 (setting the window comparators higher voltage) is reference via a zener stabilised  BJT Q1 setting the voltage on Q2 emitter at 5.6 volts.  The purpose of the resting 5.6V on the differential pair emitters is to cut off further changes to IC2 pin 5 at low supply voltage.  A little tricky! (see more below) Q2 is the output voltage error amplifier driving / setting pin 5 voltage control on the second 555. This has the effect of increasing the voltage of the pulse on pin 3.  Note the Q1 base emitter is reverse biased in the normal operation range of the supply (DC input >13V which gives about 25.5V out).

The design output voltage is approximately 2 times the value of the input voltage.  I’ve tested this up to 30V input and it gave 60V output.  The tracking between 12.5 Volts input to 30 Volts input is excellent, showing 25V to 60V out.  It runs at about 23kHz.  Note, this is not pulse width modulated above 12.5V DC input.  The pulse width remains quite uniform throughout the over 12.5V DC input operation.  The output voltage of the uniform width pulse from Pin 3 of IC2 is increasing as the input voltage is increased to the supply.  This is to drive more current to switch the BD139 transistor at higher output voltages and maintain linearity and tracking.

From DC input voltages of 7V to 12.5V approximately, there is pulse width modulation employed and the output voltage is again approximately 25 Volts.  The On-time for the BD139 is much longer duration at these lower input voltages.  How this works is because the pseudo differential amplifier (emitter follower driving Q2) with the constant voltage of 5.6V in it’s emitter circuit becomes active in this operational area.  A much larger change is seen at pin 5 on IC2, lowering that voltage and therefore increasing the pulse width at pin 3.  This ensures that the inductor has more charge time, and therefore gives a higher output voltage under these low input voltage conditions.

Had the input voltage been fixed and then a variable boost required, then the duty cycle of the pulse to the BD139 switching transistor would need to vary using PWM throughout it’s operation.  In this design though, the output tracks the input with a multiplication factor of 2X.

This is a tricky circuit to get our heads around.


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