how to analyse the current sense circuit

[sissi]

hi
there is a current sense circuit of boost converter. but i dont know the theory. M0 is the power transistor while M1 is a sensing transistor,the size ratio of m0 to m1 is k.my question is the function of Q2 Q3 and M5 M7? it is reasonable that i think the circuit as a current to voltage amplifier?

 

[Btrend]

hi
there is a current sense circuit of boost converter. but i dont know the theory. M0 is the power transistor while M1 is a sensing transistor,the size ratio of m0 to m1 is k.my question is the function of Q2 Q3 and M5 M7? it is reasonable that i think the circuit as a current to voltage amplifier?

1. Q2, M4 are used to bias M7
2. M3 is a common-gate amplifier, Q3 is used to boost the output impedance of M3, and this impedance will in parallel with M5's Rds. This make the output impedance large. so the gain will increase too.
3. M2 is biased in triode region, which act like a resistor, such that it can convert
current to voltage to input M3. whenever this current is large, it will make VSEN high to indicate over current .

But I don't know why BJT is used in this circuit ? why not NMOS ? maybe some
application issue.

 

[liuyonggen_1]

I think, m3 also operate in linear region to keep the drain voltage of m2 small for the current accuracy between m1 and m0.
usually, the threshold voltage of BJT is lower than that of MOS transistor. if using nmos transistor to replace Q3, maybe,you need another biasing circuit.

in fact, it's current comparator.

 

[sissi]

Hi,liuyonggen_1

why is it a current comparator? could you explain clearly?

Reguards
sissi

 

[liuyonggen_1]

Hi,liuyonggen_1

why is it a current comparator? could you explain clearly?

Reguards
sissi

voltage comparator is the comparison between two volatges
current comparator is the comparison betwwen two currents such as betwwen the current reference and the operating current of the power mofet.

 

[sissi]

Hi,liuyonggen_1
This circuit is used for current mode converter.if it is a current comparator, how to ensure the accuracy?
sissi

 

[liuyonggen_1]

Hi,liuyonggen_1
This circuit is used for current mode converter.if it is a current comparator, how to ensure the accuracy?
sissi

usually, every dc-dc converter has a voltage control loop. this loop gain determines the magnitude of the output voltage ripple.

current mode has a better response speed than voltage control.

 

[sissi]

Hi,liuyonggen_1
Maybe you mistook me. i mean the accuracy of the current sensing circuit. This current decides the ramp compesation.

Reguards
sissi

 

[liuyonggen_1]

Hi,liuyonggen_1
Maybe you mistook me. i mean the accuracy of the current sensing circuit. This current decides the ramp compesation.

Reguards
sissi

In fact, current comparator is also a voltage comparator.
the accuracy of the current comparator is determined by the output resistance. you could improve the accuracy by using cascode stucture.

Added after 2 minutes:

Hi, sissi, do you show me the structure of slope compensator in your boost converter project?
thanks a lot!

 

[johnsmith101]

Where is this circuit from? IEEE paper? If so, can you upload it. Thanks.