What current sense methods are used in switching regulators today? What are the advantages and disadvantages of each method?
There are four current sense methods used in switching regulators today. These are:
a) Resistor sense
b) Current transformer
d) DCR of output inductor
Resistor Sense: In the Resistor sense method, a low value resistor is placed in series with the inductor current path (either in series with the output inductor or in series with the switching element). The voltage across the resistor is then used as a current indicator signal.
Advantages: It is very precise (precision as good as the tolerance and temp variation of the resistor itself), it is simple to understand and implement
Disadvantages: In medium to high current applications, higher power rating and low value resistors are required. They tend to get expensive and take up PCB real estate. The resistors consume power, hence the efficiency is compromised.
Current Transformer: The inductor current is sensed by placing the primary winding (which is usually of one turn) of the current sense transformer in series with the switching element. The secondary of the current sense transformer is loaded with an appropriate value resistor (through a diode) to get the right signal level.
Advantages: Low loss current sensing, easy to scale the current signal (simply change the resistor on the secondary side) to any level, isolates the power circuit from the low level signal stuff, precise.
Disadvantages: Takes up a significant PCB real estate, very expensive to implement
Rds(on): The inductor current is sensed either during Ton or Toff portion of the switching cycle by monitoring the voltage across either the high-side or low-side (synchronous buck) switch.
Advantages: It is free (since Rdson is an integral part of a MOSFET), does not take up extra PCB real estate, does not burn up extra power associated wiht the previous two methods above
Disadvantages: It has a large temperature coefficient (causes loop gain variations in current mode regulators relying on the current sense gain provided by Rds(on)), has high initial tolerance (could be +/-30% or more). Hence, overall accuracy is poor.
DCR: In this method, the voltage drop across the DC resistance of the output inductor is sensed by placing a series RC across the inductor terminals and the voltage across the "C" is sensed. With correctly selected RC's, the voltage across the C accurately represents the voltage across the DCR of the inductor.
Advantages: Lossless current sensing with reasonably good accuracy (initial accuracy can be as tight as +/-5% depending on the inductor, although DCR does have temperature coefficient of approximately 0.4% per degree C), takes up a very small PCB area, provides continuous output current monitoring
Disadvantages: Requires a high common-mode tolerant current sense amplifier to process the current information, maximum Vout is limited by the maximum common mode voltage of the current sense amplifier inside the controller
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