Schottky diodes

Synchronous rectification!

John

John Larkin is right - you really do need to think about synchronous rectification. A decent-sized power MOSFET should offer a much lower forward voltage drop than 0.25V.

That would be 50 milliohm of channel resistance. You can do an order of magnitude less than that in a TO-220 packaged MOSFET for a couple of dollars, if the Farnell catalogue is anything to go by. They come with a big gate and lots of gate capacitance to drive, but it would solve the powr dissipation problem.

-- Bill Sloman, Nijmegen

Just use a sync buck -> no diode. Plenty of driver chips for that available these days.

--
Regards, Joerg

http://www.analogconsultants.com/

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Use another domain or send PM.

OK this is a different project (this one is lower power) but when I was fault finding a synchronous rectifier in a buck converter:

John Larkin - 18 Feb 2010

/quote

Synchronous rectifiers are often not worth the hassle at output voltages and currents like this. Try shunting Q3 with a schottky, or replacing it entirely.

John

/end quote

Bill Sloman wrote in news: snipped-for-privacy@r29g2000yqj.googlegroups.co m:

In order for synchronous rectification to be worth it you need a FET with about 10 to 15 moHm RDSON @25C preferably 10moHm or less.The fet conduction losses are the RMS current its subject to not the 5A output current then their is switching losses and losses associated with the drive circuit.

Considering his diode is a 100V device FETS with 10-15 moHm RDS are slim to non-existant with 100Vds. The closest I have is a panasonic device with Vds of 80V IN lfpak.

The only thing about a low Vf schottky is they usually have higher reverse current; depending on the blocking voltage it can add to the losses.

Hammy wrote in news:Xns9E34453556394Hammyhamsterca@69.16.185.250:

@r29g2000yqj.googlegroups.

Heres how to estimate diode losses from vishay.

Pd = Forward power loss = IF(AV) x VFM at (IF(AV)/D) PdREV = Inverse power loss = VR1 x IR (1 - D); IR at VR1 = 80 % rated VR

So with D=0.5 and a reverse current of 8mA and blocking voltage of

80V the reverse losses are about 0.32W alone.Using typical values from your Sanyo part.

Thanks for that. I missed the reverse leakage current.

The Sanyo part is 60V rated and I chose it because I am already using it on another project. My max supply is 32V. So VRI will be 4mA. Output voltage is 8.4V so the diode is reversed for about 25% of the time.

So Pdrev is about 0.032W.

I'm adding a smaller, low capacitance, diode and a 100nH before the main schottky to reduce the effect of the diode's capacitance during switching. This should improve RFI and efficiency.

If you are accusing me of saying that synchronous rectifiers make sense in some situations, and don't make sense in others, consider me guilty.

John

It wasn't an accusation. I was just pointing out I had been following your good advice;)

Last time it was 12V/8A or 24V/5A switched and it was hassle to get the sychronous rectifier working properly with a chip designed for the job.

This time it is 8.4V/5A and I chose a simple switcher, LM22677. All works well except for the diode dissipation. It is a bit late to change to synchronous which is why I am looking at optimizing the diode.

I posted because running a part stinking hot to achieve efficiency goes against my basic instincts to keep everything as cool as possible.

What's your input voltage? Assuming enough output inductance, the diode dissipation is about Io*Vf*duty cycle, which shouldn't amount to a lot of efficiency loss at 42 watts out. At 50% duty cycle and 0.3 volts drop, that's only 0.75 watts in the diode. Maybe you just need a little more heat sink. Or let it get hot.

A TO-220 can be heat sunk to a bit of copper pour and dissipate a watt easily. I posted on a similar situation recently. You can dump heat into connected pours, or transfer heat through the pcb laminate into a bigger plane, like the ground plane.

Here's 1 watt, cooled by conduction through 25 mils of FR4 to the copper on the opposite side, electrically isolated:

ftp://jjlarkin.lmi.net/T750_t1.JPG

ftp://jjlarkin.lmi.net/T750_t3.jpg

ftp://jjlarkin.lmi.net/T750_t5.jpg

On the final layout, we had the big ground plane on layer 2, and added added a heat spreader on layer 4 to better reduce the hot-spot effect close in. You shouldn't have to work very hard to dump 3/4 of a watt.

There are also cheap u-shaped heat sinks for use with TO-220s on a pc board.

John

The max in is 32V, nominal 24V. I'm using more inductance than I need at 8.4V, 33uH, because the unit can do 8.4V/5A or 16.8V/2.5A.

National web bench suggests a schottky with 0.7V Vf which gives a calculated Pd of 2.5W. Don't tell me web bench is crap, I know but it is a quick tool to get to the starting point. It calculates the SMB package diode Tj as 30C at 2.5W!

As I originally posted a TO220 in air should be fine. I'm allowing for a small heat sink but don't think I'll need it. I prefer not to dump the heat into the board as the IC and inductor will be close by and putting heat in as well.

the

e

I recently had the same dilemma and the same trade-offs (for reverse- voltage protection, not a SMPS). Running hot was tempting, but I ultimately went with a FET.

OTOH, old power-electronics hands advise "If it's not hot, you're wasting silicon."

-- Cheers, James Arthur

I couldn't get the chosen schottky in quantity and all the alternatives I could find didn't have low enough Vf with low enough reverse leakage at high temperature.

So I had to redesign for a synchronous rectifier and used the LM3150 which gave me problems on an earlier design. The redesign works excellently using a pair of Infineon BSC093N04LS power SO8 FETS with a schottky across the lower FET. The efficiency of the converter exceeds

95%. I'm loosing more power in protection FETS, fuse, and input reverse protection.

I've always used 2 layer boards but over the last few months moved to

4 layer boards for power conversion. It has dramatically reduced design time as things just work. Top layer, 2oz, and 1 internal layer for tracking, an internal layer for power planes and the bottom layer, 2oz, for ground and power dissipation.

The overall efficiency is >90% compared to

Be careful if you do that. There's a short non-shoot-through period when both fets are off, and some body diode is furnishing the load current. Some of these diode have delusions of being SRDs.

The results can be impressive. Imagine every opamp on the board, even ones far away, suddenly developing 100 millivolts of input offset.

LM3102:

ftp://jjlarkin.lmi.net/SwitcherRise.JPG

John

I fitted a 1A schottky across the synchronous FET. That handles the current until the FET switches. No sign of overshoot.

But you just reminded me of something. I allowed for a snubber network of 33R and 1nF, a hangover form the earlier circuit, and I haven't checked performance with that disconnected.