High side current monitor with latch
in [Design]
|
From: Joerg on 22 Mar 2010 11:18 oparr(a)hotmail.com wrote: >> Is there a uC driving this (via a transistor)? Does it have a free port >> pin to reset a latch that comes up in a random state when powering up? > > Nothing like that. It's a power management circuit in a "black > box" (BB) containing voltage sensitive modules. The BB has no power > supply, this is responsibility of the end-user. Hence the need for the > circuit to provide overvoltage and reverse polarity protection. That > is already implemented and works fine. ... That's the circuitry I was asking for. It makes most sense to find a solution that uses your other parts as much as possible, introduces only a low number of extra parts. > ... BB connections to the outside > world are DC power in and module power out. > > As already indicated, the switch in the PM circuit is a P-MOSFET, it's > turned off when an overvoltage condition is detected. I then decide to > expand this to overcurrent and short circuit protection as well. > Getting that to work for all voltages in the range 12V to 40V, using > discretes, is a challenge so a high side current monitor and > comparator approach is being investigated. > > To be honest, overcurrent and short circuit protection for the MOSFET > is not an absolute necessity since each BB internal module already has > that on the ouput. So if the end-user shorts the ouput of any of the > modules, it will not impact the MOSFET. > Many ways to implement that. One is a latch made of two Schmitt inverters that gets triggered via a BJT across a sense resistor. Supply can be simple if you use a CD40106 since it's happy with anything between 5V and 15V, let it ride with VDD on your positive supply and GND on a divider, maybe plus a zener across it. A challenge would be the turn-on, meaning the situation where its supply is less than 3-4V. Also also something must reset on power-up (can be done with a reset chip, for example). The CD4093 is another candidate here. Typically you must have an UVLO in such a circuit anyhow because a FET won't reliably turn on with only a few volts on the gate. If current is drawn in that condition ... phsss ... *POOF* -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: oparr on 22 Mar 2010 16:18 Joerg, thanks for your time and advice. >That's the circuitry I was asking for. It makes most sense to find a >solution that uses your other parts as much as possible, introduces only >a low number of extra parts. I hear you...Make use of volume discounts, price breaks and lower assembly costs (less reels) as much as possible. >Many ways to implement that. One is a latch made of two Schmitt >inverters that gets triggered via a BJT across a sense resistor. Started off with high side PNP BJT and sense resistor with low side NPN BJT as latch. Same transistors are used for overvoltage protection in conjunction with TL431 voltage reference. Just threw in the sense resistor (.02 ohms). Thought O/C and S/C protection would be a piece of cake. O/C and S/C worked fine for voltages above 20V but failed miserably for voltages below 15V. Biasing the NPN to latch for a wider voltage range made it too heat sensitive for O/V latching. There is one other thing I want to try before using other ICs...Using another TL431 as both UVLO and latch on the low side of the voltage divider MOSFET drive. This TL431 would be monitoring MOSFET drain voltage and hence the unit will always poweron with the MOSFET off. A momentary push button switch is needed to turn it on. Similary, drive to the MOSFET will be removed the moment drain voltage falls below 10V. This should also enhance S/C protection since the drain voltage will fall below 10V during a S/C. I say enhance since the high side PNP BJT will also be pulling the gate up to the source during O/V, O/C and S/C conditions. There is just one caveat, Vca for the TL431 will exceed its Vca max of 36V when operating above 36V and drain voltage falls below 10V (Ioff state). Any attemp to divide the low side voltage further will bring Vgs close to Vgs min when the MOSFET is supposed to be off. Any suggestions as to how to address this? Thanks. On Mar 22, 11:18 am, Joerg <inva...(a)invalid.invalid> wrote: >
From: Joerg on 22 Mar 2010 16:46 oparr(a)hotmail.com wrote: > Joerg, thanks for your time and advice. > >> That's the circuitry I was asking for. It makes most sense to find a >> solution that uses your other parts as much as possible, introduces only >> a low number of extra parts. > > I hear you...Make use of volume discounts, price breaks and lower > assembly costs (less reels) as much as possible. > >> Many ways to implement that. One is a latch made of two Schmitt >> inverters that gets triggered via a BJT across a sense resistor. > > Started off with high side PNP BJT and sense resistor with low side > NPN BJT as latch. Same transistors are used for overvoltage protection > in conjunction with TL431 voltage reference. Just threw in the sense > resistor (.02 ohms). Thought O/C and S/C protection would be a piece > of cake. > > O/C and S/C worked fine for voltages above 20V but failed miserably > for voltages below 15V. Biasing the NPN to latch for a wider voltage > range made it too heat sensitive for O/V latching. There is one other > thing I want to try before using other ICs...Using another TL431 as > both UVLO and latch on the low side of the voltage divider MOSFET > drive. > > This TL431 would be monitoring MOSFET drain voltage and hence the unit > will always poweron with the MOSFET off. A momentary push button > switch is needed to turn it on. ... Some users might not like such a button. > ... Similary, drive to the MOSFET will be > removed the moment drain voltage falls below 10V. This should also > enhance S/C protection since the drain voltage will fall below 10V > during a S/C. I say enhance since the high side PNP BJT will also be > pulling the gate up to the source during O/V, O/C and S/C conditions. > > There is just one caveat, Vca for the TL431 will exceed its Vca max of > 36V when operating above 36V and drain voltage falls below 10V (Ioff > state). Any attemp to divide the low side voltage further will bring > Vgs close to Vgs min when the MOSFET is supposed to be off. Any > suggestions as to how to address this? Thanks. > 36V is the end of the rope for a TL431. But considering that O/C doesn't work below 15V maybe you should really think about putting a CD4000 series latch in there, made from a Schmitt devices. They are really cheap. Haven't drawn it up in my head but that should work down to 5V supply, probably even less. If the FET is logic level even a 74HC Schmitt device could work but the CD4000 would make it easier to add in UVLO functionality. All this assuming you don't need super-fast switching. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: oparr on 5 Apr 2010 20:02
> 36V is the end of the rope for a TL431. But considering that O/C doesn't > work below 15V maybe you should really think about putting a CD4000 > series latch in there, made from a Schmitt devices. Done! Perfected! No current sense resistor required, no UVLO, no high side current monitoring IC, no ICs at all. Only a single BJT. A "tour de force" in smarts. On Mar 22, 4:46 pm, Joerg <inva...(a)invalid.invalid> wrote: > |