DC to AC converter

[Simon]

well I've not tested yet but surprise surprise i asked our buyer at work for 10 IRL540N and I got them next day, these have a max gate threshold of 2V so will be well turned on by 5V, even  the IRF540N did very well on 5V so with a proper logic level mosfet there should be no more triggering problems.

The board will not have large dissipative pads, if your talking surface area to dissipate heat from the TO-220 case has more of that than a tiny SOT23 package with a half inch of 0.64mm trace connected, in any case at the moment the losses seem to be very very low as I've run the actual pump and the mosfets were not even warm in fact still felt cool to touch.

[Zero999]

well I've not tested yet but surprise surprise i asked our buyer at work for 10 IRL540N and I got them next day, these have a max gate threshold of 2V so will be well turned on by 5V, even  the IRF540N did very well on 5V so with a proper logic level mosfet there should be no more triggering problems.

It's not that simple, you need to ensure that the MOSFET will pass the required, for exmaple, according to the IRL540 datasheet, if the gate voltage is 2.5V, the drain current will be no more than 5A to 7.3A depending on the temperature, at which the on voltage will be unacceptable.

Note that the graph on the datasheet is for a typical MOSFET and the currents are higher at higher tempertures. The minimum and maximum gate threshold is 1V and 2V respectively, so a typical MOSFET will be about 1.5V so add 0.5V to the V_DS on the graph for the highest threshold and subtract 0.5V for the lowest.

[Simon]

The gate of the N channel mosfets will be getting 5 V (4.5 at least) so more than 7A will be available and all i need is 0.6 so there should be no problem ?

[Zero999]

Yes, what you're doing is fine, with 5V at the gate, the MOSFET will be biased fully on with low conduction losses.

I didn't mean to say that there would be a problem, just that you should be wary of paying too much attention to the threshold rating which is specified at a very low current: 250µA in this case. In other words, if you have a 1A load and put just 2V on the gate, it will get hot.

EDIT:
Below the saturation current, the actual current drawn will depend on the load, not the gate voltage, which is what you want.

[Simon]

yes i realize that the Vgs threshold is just a start point, the IRF model works well on 5V with a threshold of 2-4V so i figured the IRL would be well biased to switch fully on.

[jahonen]

Generally, if there is a Rds(on) figure for a gate-source voltage in the datasheet, then it usually means that the voltage is large enough for "low-ohm region" operation.

Regards,
Janne

[Zero999]

It's normally specified at 10V but logic level MOSFETs are specified at 5V and some low voltage devices at 3V.

[TechGuy]

The board will not have large dissipative pads, if your talking surface area to dissipate heat from the TO-220 case has more of that than a tiny SOT23 package with a half inch of 0.64mm trace connected, in any case at the moment the losses seem to be very very low as I've run the actual pump and the mosfets were not even warm in fact still felt cool to touch.

Thats not entirely correct. the SOT23 soldered to a PCB will have a large area to dissipate heat into. It will dump thermal energy into the PCB board. A TO-220 MOSFET package without a heatsink is a disaster in the making.

As far as your test runs, don't assume that the conditions you experienced with your test setup will apply to all pumps and situations. What if the Power supply is 10% to 15% higher or lower voltage?  What happens if the pump is brand new or old and has a different current load then your test pump? Lab conditions rarely are the same as real world conditions. An Engineer designing production system will always consider situations that are outside of normal/idea conditions.

I don't know why you're using TO-220 instead of surface mount devices. SMT are cheaper, use less PCB real estate (smaller PCB saving $$$) and are cheaper to mount (via pick and place equipment), saving production costs.

[Simon]

I don't know why you're using TO-220 instead of surface mount devices. SMT are cheaper, use less PCB real estate (smaller PCB saving $$$) and are cheaper to mount (via pick and place equipment), saving production costs.

Because i have to make it.....idiot proof. My company are pretty good at going and finding the worst cowboy manufacturers against all sane advice and that they use one that is SMD capable is going to be down to pure luck. And I am working under and ever more senile engineer that knows nothing about electronics and how to spec electronics and packages so I'm not getting all of the information I'd like about how it will be manufactured and how he plans to inevitably screw it all up during assembly.

At the moment we are in "consideration stage" they just want to see that it is possible. I did suggest today that a D-pak is used for the mosfets as this will sit flat with the board (and is what the pumps original controller uses) I know conditions will vary that's why I have a dissipation of less than 200mW from all four mosfets combined that's about as robust as you will get. He is now looking into a case to put it in an once he has got one i will do some thermal tests.

I can't use SMD until I am assured that the maker he wants to use (who is probably making them shacked up in a barn somewhere) even knows what SMD is, remember to this lot electronics is a whole new scary world, they still use high power resistors on heat sinks to drop 24V to 12V on air con compressor clutches. I've suggested that we (I) make them in house but my lot are so damn scared of making anything as having a supplier to blame comforts them but having seen the last disaster which I'm sure they will not recover their costs from I'm not too sure that's the way to do it.

Again we are just looking at possibilities at the moment and all this needs to go before the MD and technical director for a final decision so maybe I can convince them to make it in house and let me do it then I know what I'm designing for and my MD loves to save those pennies, that's why the whole place is the cockup it is.

[Thermal Runaway]

Naturally my first thoughts fell to a PIC but i will have to "manually" program the 50 Hz square wave in as on a 4MHz clock the lowest PWM output is 245 Hz unless of course there is a pic with less than 4MHz internal clock ? 12F683 maybe ?

I've nothing further to add regarding your PSU design, there are others on here already giving better answers than I would give, but regarding your PIC internal clock question, the internal oscillators can usually be programmed to work at a whole range of different frequencies via an internal divider.  It's possible to program an internal clock to operate as low as 32KHz.  There's a table of possible internal clock values in the datasheet and these are set up via the PIC internal registers.

However, assuming you were using a PIC to generate a square wave output; lowering the clock frequency of the device would not necassarily be the best solution to your problem.  It would be better to leave the clock alone so that the rest of your PIC application runs quickly and then use time wasting methods (delay routines) to generate your output at the required frequency. 
Better still would be to use an internal timer which can be configured to run on a divided down version of the clock frequency.  When the timer overflows an interrupt is generated, and this interrupt could be used to change the state of an output pin.  You could calculate the value of prescaler to use for the timer module based on your chosen internal clock frequency (taking into account that each instruction cycle takes 4 clocks anyway) and your desired output frequency.

Brian

Brian

[Simon]

Well for the clock I simply solved it by using delays and setting pins high and low, what my point was that with a 4MHz clock the slowest PWM output is 245Hz which was way to fast so I had to resort to manually creating the waveform