Re: Do I have to measure the phase of the current?

bernhard.kramer@xxxxxxxxxxxxxxxxx wrote...
>
> I am working on a current regulating circuit where the phase
> of my current is important. To be more precise:
> I have the following circuit:
>
> - Vref
> |
> \ OPA227 +--------------+
> R1 / | |
> \ |\ | |
> | | \ | |
> +---------|- \ |++ |
> | | >----|| 2SK3680-01 ---
> \ +-----|+ / |++ - Battery
> R2 / | | / | ---
> \ | |/ | -
> | | (GND) | |
> +--------------------+ |
> | | |
> | / |
> | R_Shunt \ |
> | / |
> | | |
> +----------------+--------------+
>
> My Vref voltage will be an AC signal up to 10 KHz with a
> certain DC offset. As I have to know (but not to control)
> the phase of my current at 3° precision, I would like to ask
> you:
>
> Do I have to measure the phase (which would impose another
> circuitry) or do I know the phase with the wanted precision
> as I am imposing it?
>
> I fear that not only the Opamp, but also the Mosfet will
> introduce a phase shift higher than 3° compared with the
> Vref voltage.
>
> Some important data: The Opamp OPA227 has a Unity Gainbandwith
> of 8 MHz and a Slew Rate of about 2.3 V/us. The Mosfet 2SL3680-01
> is a power Mosfet, but I don't know too much about what criterias
> might be important. It seems (if I interpret the data*** correctly)
> that the Cgs is not more than 10 nF.
>
> The Gain of the Opamp never exceeds 10. The AC-signal is supposed to
> be never higher than 2 Vpp.

As you noticed, the MOSFET's high gate capacitance wil be trouble
for the opamp. I've edited your circuit to show the standard way
to deal with this problem, isolating the opamp from the FET with a
series resistor, and providing direct high-frequency feedback.

> - Vref
> | OPA227
> \ Cf +--------------+
> R1 / ,---||----, | |
> \ | _ | | |
> | | | \ | | |
> +-----+---|- \ | Rg |-+ |
> | | >-+--/\/\--|| 2SK3680-01 ---
> \ +-----|+ / |-+ - Battery
> R2 / | |_/ | ---
> \ | | -
> | | (GND) | |
> +---------------------------+ |
> | | |
> | / |
> | R_Shunt \ |
> | / |
> | | |
> +-----------------------+--------------+

Although the Rg Cf components stabilize the opamp feedback loop,
they limit the ability of the circuit to work accurately at high
frequencies. One solution is to use an opamp that has a lower
open-loop Zout, or even better, use a power buffer between the
opamp and Rg, allowing you to reduce the value of both Rg and Cf.
E.g., http://focus.ti.com/docs/prod/folders/print/buf634.html

In this manner you should be able to achieve < 3 degrees at 10kHz.


--
Thanks,
- Win
.

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