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| High side current sense using differential opamp |
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| npelov:
--- Quote from: Hero999 on June 22, 2018, 07:55:01 am ---Yes, that circuit can go lower but is it stable? As I said previously, it's got a lot of gain. I notice you've got C1 to provide some frequency compensation but is it enough? I suspect not. It's extremely likely that circuit will oscillate. --- End quote --- I have no idea if it will be stable. Having mosfet in the loop is always tricky. I thought the low input capacitance of 2N7002 would help to stabilize. --- Quote from: Hero999 on June 22, 2018, 07:55:01 am ---Have you thought about using an op-amp which can work up to the positive rail, in a current mirror configuration? The op-amp only needs to work up to 5V and 5V rail-to-rail op-amps are cheap. The P-MOSFET could even be swapped for a PNP BJT, which will reduce the accuracy slightly, due to the base current, but it's chaper and possibly make it more stable, since it has less capacitance, than the MOSFET. --- End quote --- That circuit is definitely interesting. I was thinking about two 12V lead acid batteries. The charger would have supply of >=30V. Most of the current sense don't go that far. And this circuit should minimize current - at higher voltages few mA means a lot of wasted power. Zero drift is not required unless precision is needed. Maybe I can use MCP601/MCP602 because its low power consumption (I don't have to burn a lot of current through Rbias and it has low offset current so R1 can be higher. Thanks! |
| Zero999:
--- Quote from: npelov on June 22, 2018, 09:28:28 am --- --- Quote from: Hero999 on June 22, 2018, 07:55:01 am ---Yes, that circuit can go lower but is it stable? As I said previously, it's got a lot of gain. I notice you've got C1 to provide some frequency compensation but is it enough? I suspect not. It's extremely likely that circuit will oscillate. --- End quote --- I have no idea if it will be stable. Having mosfet in the loop is always tricky. I thought the low input capacitance of 2N7002 would help to stabilize. --- End quote --- The input capacitance of the MOSFET, will only make oscillation more likely, as it introduces an additional phase shift. The biggest problem is the gain of the MOSFET, increases the total gain to above unity, when the op-amp's phase shift is 180o, causing oscillation. --- Quote --- --- Quote from: Hero999 on June 22, 2018, 07:55:01 am ---Have you thought about using an op-amp which can work up to the positive rail, in a current mirror configuration? The op-amp only needs to work up to 5V and 5V rail-to-rail op-amps are cheap. The P-MOSFET could even be swapped for a PNP BJT, which will reduce the accuracy slightly, due to the base current, but it's chaper and possibly make it more stable, since it has less capacitance, than the MOSFET. --- End quote --- That circuit is definitely interesting. I was thinking about two 12V lead acid batteries. The charger would have supply of >=30V. Most of the current sense don't go that far. And this circuit should minimize current - at higher voltages few mA means a lot of wasted power. Zero drift is not required unless precision is needed. Maybe I can use MCP601/MCP602 because its low power consumption (I don't have to burn a lot of current through Rbias and it has low offset current so R1 can be higher. Thanks! --- End quote --- I forgot to mention that one downside to that circuit is the output impedance will be equal to RL, but that won't be a problem if the load impedance is very high or a buffer could be added. Another possibility is an op-amp with P-channel J-FET inputs, such as the TL072, which will work with the inputs at the positive supply rail. Although do take note that this isn't a guaranteed specification, so is probably not suitable for mass production. http://www.ti.com/lit/ds/slos080n/slos080n.pdf |
| npelov:
--- Quote from: Hero999 on June 22, 2018, 10:52:09 am ---The input capacitance of the MOSFET, will only make oscillation more likely, as it introduces an additional phase shift. --- End quote --- And that's why it's good that it's as low. But the gain ... could it be fixed by rising the integrating capacitor. I'm interested because I had a lot of problems with constant current load with a mosfet I thought gate charge is the only problem. How can you drive a mosfet with an opamp without without oscillation? Any requirements to the opamp? Any components to reduce the mosfet gain? The current mirror is just fine. In this case I'll feed it to PIC ADC input which needs something with input impedance <=10k. And if I have to buffer it after it's amplified it's not that big deal - I can do it with any opamp. It's not like it'll multiply offset of 1mV by 100. Well maybe it's good to have lower bias current - like MCP601 I mentioned. The problem is that I can't Use dual opamp (MCP601) because the supply of the first opamp is lifted off the ground. And if I have two high side shunts at different voltages, again I have to use separate chips. |
| Zero999:
--- Quote from: npelov on June 22, 2018, 12:01:56 pm --- --- Quote from: Hero999 on June 22, 2018, 10:52:09 am ---The input capacitance of the MOSFET, will only make oscillation more likely, as it introduces an additional phase shift. --- End quote --- And that's why it's good that it's as low. But the gain ... could it be fixed by rising the integrating capacitor. I'm interested because I had a lot of problems with constant current load with a mosfet I thought gate charge is the only problem. How can you drive a mosfet with an opamp without without oscillation? Any requirements to the opamp? Any components to reduce the mosfet gain? --- End quote --- The problem is, the MOSFET is in common source configuration, which has a high gain. It would still oscillate, if it were replaced with a BJT, which could even make matters worse, since it has even more gain. Increasing the value of the gate resistor and adding another resistor between the gate and source, would help to reduce the gain to a more manageable level. --- Quote ---The current mirror is just fine. In this case I'll feed it to PIC ADC input which needs something with input impedance <=10k. And if I have to buffer it after it's amplified it's not that big deal - I can do it with any opamp. It's not like it'll multiply offset of 1mV by 100. Well maybe it's good to have lower bias current - like MCP601 I mentioned. The problem is that I can't Use dual opamp (MCP601) because the supply of the first opamp is lifted off the ground. And if I have two high side shunts at different voltages, again I have to use separate chips. --- End quote --- You could use a dual op-amp IC for the two high side shunts and another dual op-amp IC for the buffer. |
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