Half Bridge Converter questions and problems - Page 1
Forum section: electronics / projects

author: SaabFAN
author_rank: Frequent Contributor
timestamp: November 08, 2016, 11:58:29 pm
content: I'm having problems with a half bridge-converter I designed according to several online-calculators and a book that explains their function and shows quite a few circuits.Up to about 1,5A output current @ 18V (Input voltage: 12V from a repurposed PC-PSU) it works as expected, but if I turn the load resistance down, the voltage starts to drop.The control-chip (TL494) is maxed out at 39,5% Dutycycle and about 350kHz oscillator-frequency and doesn't increase the duty-cycle anymore. Vdd for the TL494: 10V.I have also observed rather strong oscillations at the midpoint of the capacitive voltage-divider that start when the voltage starts to drop and the TL494 cannot give any more duty cycle.The driver-circuit for the MOSFETs is working by the way. There's enough amplitude to turn both N-Channel FETs fully on.Adding a capacitor with 2200µF at the input seems to improve the performance a bit.My questions:- What capacitor-value should I use for the capacitive voltage divider connected to the transformer? I have found a lot of information about the transformer and all the other components, but how to calculate the capacitor values of the divider remains a mystery. Am I right to assume that I need enough capacitance to keep the voltage at the midpoint around 1/2Vin +/- 2V at full power (about 80A for about 1,8µs)?- I'm using an ETD34 core with the coils made out of cables that I salvaged from a PC-Power Supply. Any idea how much this could affect the performance of the transformer? For a second attempt I have build another transformer with 3 turns of 2,5mm thick copper-wire on the primary side and 18 turns 0,8mm copper-wire on the secondary side.Also, does it make a difference how the coil is wound? Is winding in only one direction allowed (from the bottom up) or can I wind the turns up and down? - I have no experience with transformers and the book about power-supplies does leave out this little detail unfortunately.- Am I asking too much of the TL494 to work at 350kHz oscillator-frequency with the Pulse-Steering FlipFlop active (Max Duty-Cycle: about 50%, output frequency: ca. 1/2 Oscillator-frequency)?Link to the CircuitMaker project:http://circuitmaker.com/Projects/Details/Thorsten-Schroeder/12V-Input-24V-250W-Isolated-Output-Power-SupplyIf I have time tomorrow, I'll post pictures of the circuit if necessary - It is the standard half-bridge converter one can see in many placces on the internet.

author: bktemp
author_rank: Super Contributor
timestamp: November 09, 2016, 12:48:50 pm
content: Quote from: SaabFAN on November 08, 2016, 11:58:29 pm80A? I have never seen a half bridge design for such a high current. For low input voltages at that power level you typically use push-pull or full bridge, because the capacitors need to handle the full current -> large capacity and very low ESR required.QuoteThe max duty cycle gets lower when the frequency is high, because of a fixed dead time. That's why you only get 40%.

author: SaabFAN
author_rank: Frequent Contributor
timestamp: November 09, 2016, 02:30:44 pm
content: I have put 3 1206 capacitors its in parallel for each leg of the divider to achieve a low enough ESR that the power dissipation is not too high. But I guess I just don't have enough capacitance...Full Bridge might have been better, but at 250W (Output 18V, ca. 15A, which required pulses of 80A through the primary winding) I thought a half bridge converter would be more suited because it halves the required amount of FETs an driver-chips.About the TL494:I was afraid of that. Is there a pin-compatible chip available that can reach higher frequencies?Gesendet von meinem Nexus 4 mit Tapatalk

author: bktemp
author_rank: Super Contributor
timestamp: November 09, 2016, 02:48:49 pm
content: Quote from: SaabFAN on November 09, 2016, 02:30:44 pmAt 80A you need quite some capacitance: 50-100uF for <2V ripple, but more important is the ESR: Even 1mohms will dissipate 6.4W at 80A.You need something like 10x 22uF ceramic capacitors in parallel.If you don't want to use full bridge, use push-pull. Push-pull doubles the voltage swing, therefore halves the current compared to half bridge. But you need a centre tapped winding, therefore it is less space efficient.

author: rob77
author_rank: Super Contributor
timestamp: November 09, 2016, 02:52:59 pm
content: Quote from: SaabFAN on November 08, 2016, 11:58:29 pmQuoteno, no and no.... that doesn't computeyou have to use several thinner wires in parallel to wind a high current winding - skin effect is a bitch, you simply can't use such a thick wire for high frequencies.

author: SaabFAN
author_rank: Frequent Contributor
timestamp: November 09, 2016, 10:16:50 pm
content: Ok, so I guess using a simple cable is also not correct and I need High Frequency Litz-Wire ("HF-Litze" in german) with isolated copper-fibers for the primary winding?

author: rob77
author_rank: Super Contributor
timestamp: November 09, 2016, 10:46:17 pm
content: Quote from: SaabFAN on November 09, 2016, 10:16:50 pmthat would be the best but several strands of thinner wire (e.g. 6 x 0.6mm) will do as well

author: SaabFAN
author_rank: Frequent Contributor
timestamp: November 09, 2016, 11:30:18 pm
content: Okay, I'll try that.That leaves the problem with the capacitive voltage divider.The most capacitance I could fit there is 60µF in each leg (2 10µF caps soldered on top of each other, 3 in parallel).If I understand the theory behind this converter correctly, the power passed through the primary winding of the transformer is important. So I need a stiff voltage divider that can sink/source the required current so the voltage doesn't change too much.I'll give my current board a last chance with that kind of configuration, but I guess I have underestimated what I need to handle these kinds of currents. Cooling is good enough, but the capacitors are a bit out of their leagueAre there any similar pitfalls with a full bridge topology? I would reuse the working control with the TL494 and drive the MOSFETs with 2 IR2110 Driver- Chips. Of course the required amount of heatsinks would double and there's the space-issue (Max. Board Space: 10x10x10cm in a box with a 80mm Fan mounted in the top), but currents would be cut in half, with less input capacitance and smaller traces needed as a result.