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1. Low-Side Shunt Resistor (classic method)

The most widely used method: a small resistor in the return path of the switch or load.

• Easy to measure with op-amp, CSA or controller pin
• Low noise compared to high-side sensing
• Allows cycle-by-cycle current limit

Limitations:

• Introduces additional power loss
• Cannot detect short to GND
• Requires Kelvin connections

2. High-Side Shunt (precision current sense)

Measures current on the positive side, enabling detection of shorts to ground. Must use a differential amplifier with high CMRR.

• Detects load shorts reliably
• Works in buck, boost and motor drivers
• Amplifier must handle common-mode voltage

Typical amplifiers:

• TSC2011 / INA240 (TI)
• MAX9938 (Analog Devices)
• ZXCT series (Diodes Inc.)

3. Current Transformer (CT)

Used in offline SMPS, PFC and gate-drive circuits for fast, isolated current measurement.

• No losses in primary (ideal for high current)
• Provides galvanic isolation
• Excellent for switch current monitoring

Limitations:

• No DC response
• Needs burden resistor
• Saturation at high load transients

4. RC-Filtered Sense (lossless current sensing)

“Lossless sensing” reconstructs current from switch voltage using RC network. Works with MOSFET R_DS(on) or diode reverse recovery profile.

• No shunt resistor → no power loss
• Very inexpensive

Disadvantages:

• Not accurate across temperature
• Dependent on MOSFET parameters
• Needs calibration

5. MOSFET Drain-Source Sense (R_DS(on))

The MOSFET itself is the shunt. Voltage across R_DS(on) is used for cycle-by-cycle control.

• Zero BOM cost
• No power loss in discrete shunt

Issues:

• R_DS(on) varies strongly with temperature (±30–60%)
• Not suitable for precise limit
• Needs filtering and blanking time

6. Hall-Effect Current Sensors (isolated)

Ideal for high current (>10 A), motor drives, battery systems and galvanic isolation.

• No insertion loss
• Works for AC + DC
• Excellent for inverters & BLDC

Drawbacks:

• Moderate response time
• Offset drift with temperature
• Larger physical size

7. Integrated Controller Sense (Peak / Valley sense)

Many SMPS controllers include current sense pins for:

• Peak current mode
• Valley current mode
• Hiccup/latched OCP
• Soft-start current foldback

These circuits usually require clean layout, RC filtering and Kelvin return to the source/shunt.

8. PCB Layout Rules for Current Sensing

• Use Kelvin sense pads on the shunt
• Keep sense traces short and away from SW node
• Route sense traces as a differential pair
• Place RC filter close to amplifier/controller
• Use solid ground plane under sensing network

9. Quick selection guide

Method	Accuracy	Loss	Bandwidth	Isolation
Low-side shunt	High	Yes	High	No
High-side shunt	High	Yes	High	No
Current transformer	Medium	No	Very high	Yes
RDS(on) sensing	Low	No	Medium	No
Hall sensor	Medium	No	Low/Med	Yes

10. Conclusion

There is no universal method for all applications. The right choice depends on:

• current range
• desired accuracy
• isolation needs
• efficiency targets
• SMPS topology

For most DC/DC converters: low-side shunt + CSA is the most practical and accurate. For high-power inverters and motor drives: Hall sensors or CTs dominate.