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Could PI device be used for 400VDC output for universal input?

Posted by: prasun on Sat, 08/02/2008
Hello Sir/Mam, Kindly update me with the following. I am looking to design output 400VDC/50mA. The input to the supply is universal . kindly suggets could we use Any PI device, If yes, which device & if not, why Not?

Comments

Submitted by PI-Chekov on Wed, 08/06/2008

Hello Prasun,

the limitation with high voltage outputs is the voltage rating of the output diode required. For a 400 V output the diode rating will be in the 1500 V to 2500 V range - you can check this by using PIXls (part of the PI Expert Suite) to do a paper design (I used a TOPSwitch-HX TOP255PN device).

The diode has to withstand the output voltage plus the additional reverse voltage induced when the primary side MOSFET turns on. If the transformer turns ratio (primary: secondary) is 1:3 then for a 375 V bus voltage (265 x sqrt 2) the addition diode stress is 1125 V giving diode minimum diode rating of 1125+400=1525 V.

The issue with a diode of this voltage rating is they have a long reverse recovery time. We normally recommend reverse recovery times (trr) <50 ns. For a 2000 V diode the trr is ~500 ns to 2000 ns.

Long recovery time causes large initial current spikes on the primary (and therefore the internal MOSFET) during start up. These typically exceed the leading edge blanking time and trigger the internal current limit, preventing proper start up.

It's been 10+ years since I tried such a design - anyone out there had success with high voltage outputs and has any tips to share?

Regards

Chekov
Submitted by prepka@behlman.com on Thu, 08/07/2008
Hello Prasun, The problem that you are having is not unique. The total output power is only 20W. The problem you are experiencing is due to many factors. Any of the PI parts that are capable of power in your rating or better should work. If regulation is not a huge issue you can try the TOP200-204. The diode recovery is a problem but you can use something like the MUR160 the current is really small the transformer ratio here is a problem. You can build several windings in the transformer and series the output at the capacitors to break the voltage stresses up. You can make 3 or 4 secondaries. The transformer should be designed to operate as a continuous mode flyback. The secondaries should be wound with heavy poly-nylon wire for dielectric strength and wound with layer of tape between them. For best operation use a good clamp on the primary as leakage inductance is a major problem as well. Spend time optimizing the transformer design and the topswitch will do the rest. Pay particular attention to the layout as this can sink you also. Read the application notes on the PI site. They have a lot of good information in them and are well written. I have dozens and dozens of successful designs using the PI parts and they work great if you do all of the homework. Good luck
Submitted by prasun on Thu, 08/07/2008
hello mr chekov, i am pleased to read your words satisfying the doubt of rectifier diode that even i had. In addition to this sir, pls let us know on what ground, on what parameter, PI device has limitation (apart from output rectifier diode) if we attempt for 400VDC output. Also,could we not try using 4 series winding of 100V in transformer secondry? also, how other designs are done for 400V output using others devices & diode selection?
Submitted by gaurangmewada on Sun, 08/17/2008
Hello Sir I am wondering we put Tranzorbs to suppress reverse primary voltage. Will it not be good in high voltage suppression? I know generally available tranzorbs won't suppress this much high voltage... But as a curiosity I would like to know what is we put 2-3 tranzorbs in series ?? Will this work??
Submitted by prasun on Sun, 08/17/2008
hello sir, i am convinced with ur explanation but one of the designer is CHALLENGING that he could try using Multiple windings at secondry using PI device. urther, he is also keen to discuss his concept with PI. so shall we discuss with you anyhow, if anyhow it holds ur attention, could we discuss about it OFF-the forum?
Submitted by tufejafif on Wed, 08/20/2008
Hello, I designed a power supply which puts out up to 540V with 100mA continously, with voltage doubler at the input for 115V. but with less power... universal input It's working with a triple DC-stack at the output, in discontinous mode with 66kHz to reduce the stress on the output rectifier diodes (1200V rating, should also work with 1000V rating). Unfortunately I can't tell you more about it, because of my company's secret policy. A few hints for implementation: softstart capacitor, output-overvoltage protection (was really useful), auxiliary output (15V) for the feedback loop. good luck ;-)
Submitted by prasun on Wed, 08/20/2008
sir, kindly let me knw where did you found application note on PI site for the same.
Submitted by Adrian A. on Sat, 02/18/2012

Hi; I realize that topic is already too old, but could be useful for other readers too.

Simply use 4 diodes in series in the secondary. The diodes must each be paralleled by a resistor of 330k-680k / 500mW, for equalizing the voltage stress, which otherwise will be distributed randomly by the internal inverse impedance of diodes. Also a clamp group of 100pF + 22kohmi should be added on each diode, for the same purpose.

Secondary of the transformer should be wounded carefully, with good insulation between each layer. If possible, to reduce at half the voltage stress on windings, at the end of the first layer a rectangle of insulation is added, the wire is returned to the beginning side of the winding, a layer of insulation is applied and after that the next layer of turns is started...and so on. (This technique i used 20 years ago when i wounded high voltage transformers for TV sets, with perfect results. Also for diode series circuit, the same.).