Whats happening is that my half bridge (bipolars with proportional drive) works well with a resistive load (a few mains voltage light bulbs) whatever the duty cycle. If I connect up the transformer (EE65, n27 ferrite, 36 turns:4+4 turns, 25kHz,windings interleaved) and a dummy load on the secondary, i get a clean waveform only near full duty cycle. With shorter duty cycle, the waveform deteriorates and the transistors become lossy. Wondering if I can let the primary side run at full duty cycle and regulate the pulse width on the secondary side.
The goal is to make a 13.8V SMPS delivering about 25A, using mostly junkbox components. I already have the transformer and output choke, but at the moment im torn between using bipolar transistors or mosfets for the switching transistors. If I use mosfets, some of the bigger types take considerable driving power and I have struggled to get a decent looking waveform; anything bigger than an IRF840 seems a problem. With bipolars and proportional drive, the drive power mostly comes from the feedback winding so they seem much easier to drive. Im intending to have the PWM chip on the low voltage side and transformer drive to the gates/bases. Im intending to use synchronous rectifiers for the secondary side due to the low voltage/high current.
Would that be for car audio stuff? Then regulation, and filtering, are not very important.
What are you shooting for?
Hi
sounds like you need a better driver for the XFMR supplying the base drive on the bipolar switches, usually a discrete totem pole is used for that. Consider Baker clamps for the main bipolar switches too.
It's normal for the PWM circuitry to be on secondary side, but you will need a dedicated housekeeping supply, isolated from the primary. edit> That's the beauty of using isolated XFMR base/gate drive, you can use the PWM error amps directly sensing the secondaries.
sounds like you need a better driver for the XFMR supplying the base drive on the bipolar switches, usually a discrete totem pole is used for that. Consider Baker clamps for the main bipolar switches too.
It's normal for the PWM circuitry to be on secondary side, but you will need a dedicated housekeeping supply, isolated from the primary. edit> That's the beauty of using isolated XFMR base/gate drive, you can use the PWM error amps directly sensing the secondaries.
Last edited:
Thanks for the suggestions, I came to that conclusion yesterday. The PWM chip is a SG3524 with totem poles on each output made of BC548/558. I connected a 22 ohm resistor across the output and the waveform collapsed just about to nothing. Im going to change to SG3525, using external totem poles if required using some better transistors. Hopefully then the 22 ohm resistor will get warm!
Im still unsure whether to use bipolars or mosfets, but am steering towards bipolars. With transformer coupled proportional drive I dont need much driving power and have the option to make the circuit self starting like the old ATX PSUs. Haven't tried the baker clamp circuit so cant comment on this as yet.
The only worry I have about making the unit self starting is that it may self-start with, say, a short on the output and self-destruct.
My order for S2000A television HOT transistors has just arrived. Can anyone advise me which is the best approach: transformer coupled proportional drive, or a baker clamp, or even a combination of both? with the proportional drive the base current will increase with more loading on the power supply, but no necessarily in the correct proportion so perhaps I may have not enough drive at one extreme and too much at the other. Will the baker clamp provide more accurate drive?. Another idea im considering is how to make the supply self starting, I did an experimental set up with a single turn round the base drive transformer, a 1.5V cell and a small capacitor, connecting all together in series with a switch makes a simple way to 'kick start' it. The base drive transformer is made with the minimum possible number of turns on a small toroid so that it saturates before the main power transformer so a few cycles of self oscillation wont hurt.
baker clamp helps with turn off times because it prevents "saturation" of the switch. sorry no, they won't fix poor drive waveforms.
You could go either way since you are using a controller. I like soft starting SMPS and good fast primary current limiting.
Post yer schematics and or reference designs if you need more ideas.
You could go either way since you are using a controller. I like soft starting SMPS and good fast primary current limiting.
Post yer schematics and or reference designs if you need more ideas.
Im going for transformer coupled proportional drive as I think its simple to implement and I have the option to make the unit self starting.
I have sorted out the problem with the poor drive waveform by using an improved driver circuit. I believe the 3525 provides soft start after tripping on a fault.
I would be interested to know how you implement your current limiting, Im thinking of using a small current transformer on the primary side of the transformer, and the 3525 provides current mode control. I haven't made the feedback system yet, im manually controlling the duty cycle.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.