RC snubber on flyback primary
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From: Jamie Morken on 9 Oct 2008 06:21 Hi, I am testing a flyback and trying to size the primary side RC snubber, without the snubber my fet blows up, I started with a 100ohm 1206 package resistor and a 47nF 1kV 1812 package capacitor, and now am using a 100ohm resistor and 10nF 1kV 1206 package capacitor, this still smokes a 1206 package 1/4 watt resistor. The flyback is low power, 10watts maximum output, and I would like to dissipate as little power in the snubber as possible for efficiency. Switching to a smaller snubber cap reduces the heat in the resistor, and also I noticed in an ltspice sim that a smaller value R also decreases the heat in the resistor, so which method is more efficient to snub voltages, dropping the capacitance value or the resistance value for an RC snubber? cheers, Jamie
From: Jamie Morken on 9 Oct 2008 07:49 Jamie Morken wrote: > Hi, > > I am testing a flyback and trying to size the primary side RC snubber, > without the snubber my fet blows up, I started with a 100ohm 1206 > package resistor and a 47nF 1kV 1812 package capacitor, and now am using > a 100ohm resistor and 10nF 1kV 1206 package capacitor, this still smokes > a 1206 package 1/4 watt resistor. The flyback is low power, 10watts > maximum output, and I would like to dissipate as little power in the > snubber as possible for efficiency. Switching to a smaller snubber cap > reduces the heat in the resistor, and also I noticed in an ltspice sim > that a smaller value R also decreases the heat in the resistor, so which > method is more efficient to snub voltages, dropping the capacitance > value or the resistance value for an RC snubber? Just a bit of an update, I removed the RC snubbers on the flybacks I am testing, and they survived, so I think my original mosfet failures were from an unrelated cause, I then put on a 100ohm/68pF RC snubber and there is a 100mW extra draw on the bench supply, with a 2watt output load. I'm not sure if this small of an RC snubber is very useful, but it seems to be a good fit (ie. low power!) cheers, Jamie > > cheers, > Jamie
From: Robert Wilson on 9 Oct 2008 08:41 Hi, Have you got a DSO to look at your gate/source waveform? In my experience the quality of the gate drive is critical to reliable operation. Also you need the voltage overshoot on the primary in order to get the output to work. You just either have to work on the layout of the PCB to minimise ringing, or reduce the speed fo the gate drive in order to to make things ring up too much. You can increase the gate resistor value to do this. Rob Jamie Morken wrote: > Jamie Morken wrote: >> Hi, >> >> I am testing a flyback and trying to size the primary side RC snubber, >> without the snubber my fet blows up, I started with a 100ohm 1206 >> package resistor and a 47nF 1kV 1812 package capacitor, and now am >> using a 100ohm resistor and 10nF 1kV 1206 package capacitor, this >> still smokes a 1206 package 1/4 watt resistor. The flyback is low >> power, 10watts maximum output, and I would like to dissipate as little >> power in the snubber as possible for efficiency. Switching to a >> smaller snubber cap reduces the heat in the resistor, and also I >> noticed in an ltspice sim that a smaller value R also decreases the >> heat in the resistor, so which method is more efficient to snub >> voltages, dropping the capacitance value or the resistance value for >> an RC snubber? > > > Just a bit of an update, I removed the RC snubbers on the flybacks I am > testing, and they survived, so I think my original mosfet failures were > from an unrelated cause, I then put on a 100ohm/68pF RC snubber and > there is a 100mW extra draw on the bench supply, with a 2watt output > load. I'm not sure if this small of an RC snubber is very useful, but > it seems to be a good fit (ie. low power!) > > cheers, > Jamie > > >> >> cheers, >> Jamie
From: Jim Thompson on 9 Oct 2008 11:21 On Thu, 09 Oct 2008 04:49:46 -0700, Jamie Morken <jmorken(a)shaw.ca> wrote: >Jamie Morken wrote: >> Hi, >> >> I am testing a flyback and trying to size the primary side RC snubber, >> without the snubber my fet blows up, I started with a 100ohm 1206 >> package resistor and a 47nF 1kV 1812 package capacitor, and now am using >> a 100ohm resistor and 10nF 1kV 1206 package capacitor, this still smokes >> a 1206 package 1/4 watt resistor. The flyback is low power, 10watts >> maximum output, and I would like to dissipate as little power in the >> snubber as possible for efficiency. Switching to a smaller snubber cap >> reduces the heat in the resistor, and also I noticed in an ltspice sim >> that a smaller value R also decreases the heat in the resistor, so which >> method is more efficient to snub voltages, dropping the capacitance >> value or the resistance value for an RC snubber? > > >Just a bit of an update, I removed the RC snubbers on the flybacks I am >testing, and they survived, so I think my original mosfet failures were >from an unrelated cause, I then put on a 100ohm/68pF RC snubber and >there is a 100mW extra draw on the bench supply, with a 2watt output >load. I'm not sure if this small of an RC snubber is very useful, but >it seems to be a good fit (ie. low power!) > >cheers, >Jamie > Back almost 30 years ago, when I was designing off-line switchers, I used a Tek current probe along with a voltage probe to plot the operating SOA on the 'scope screen. Learned a lot about designing snubbers that way, and made a lot of high efficiency stuff. ...Jim Thompson -- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine Sometimes I even put it in the food
From: Eeyore on 10 Oct 2008 02:21
Jim Thompson wrote: > Back almost 30 years ago, when I was designing off-line switchers, I > used a Tek current probe along with a voltage probe to plot the > operating SOA on the 'scope screen. Learned a lot about designing > snubbers that way, and made a lot of high efficiency stuff. Lots of app notes around IIRC on snubber design too. Graham |