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Low Side RDS(on) Current Sensing

Motor drive applications with totem pole outputs often only need to sense current in the low side switch(es) of their phase(s). Including a shunt and an amplifier is a simple and obvious solution, and is really preferred if high accuracy is the objective. In those applications that only require relaxed accuracy a dedicated shunt takes a significant amount of board space and cost. The solution the readily presents itself is to use the RDS(on) of the lower MOSFET in lieu of a shunt to measure current. The initial value of RDS(on) can be subject to wide variation as well as having a large temperature coefficient (tempco) but often the digital complexity already required by these systems can be readily applied to calibration and compensation.

While there are many articles that discuss the tradeoffs and merits of high, low, and phase sensing, this article only discusses low side RDS(on) due to its demand, simplicity, and low cost.

Basic Design Concept Using an Analog Switch

When using a shunt in a low side totem-pole as in Figure 1, note that the waveform follows that of the current flow in the switched MOSFET. You can configure an RDS(on) sensing circuit to provide essentially the same waveform (minimizing the changes needed to the monitoring system).

The easy part is when the MOSFET is on, that is the voltage drop we seek. The problem occurs when the MOSFET turns off and the voltage then rapidly shoots up to nearly the supply voltage of the circuit, this same voltage would have dropped to near zero when using a shunt.

The solution proposed here is nothing more than an analog switch. The process of incorporating the analog switch revealed that the switch used, the FSA4157, had several attributes ideally suited to simplifying this application (as do a number of analog switches).

That big voltage excursion seen when the MOSFET turns off is the first example of such a problem and its solution. A simple, obvious solution would a resistor in series with our measurement circuit and a clamp, which could have been a simple diode to ground assuming we are measuring signals less than around 0.5 volts. The need for the diode was eliminated by close examination of the FSA4157 data sheet Absolute Maximum Ratings DC Switch Voltage Specification. Like many IC's this specifications was to stay within +/- 0.5 volts of the positive supply rail (a diode drop, because it is a diode). The really useful specification was for DC Input Diode Current which aside from specifying 50mA, allows one to ignore exactly what the DC Switch voltage is along as that current value is not exceeded. This is “legally” turning on the built-in diode clamps between the inputs and the analog switch supply rails.

So all you need to do is select series input resistors that respect this 50mA current rating based on the supply voltage (to the drain of the upper MOSFET). This clamping also comes in handy for the control signal that will often come from a MOSFET with 10 volt gate drive. Again, add a series resistor based on not more than 50mA at the highest expected gate voltage. One little detail regarding this clamping, it assumes the power supply pin of the analog switch can absorb the clamping current. If there is any doubt a Transient Voltage Suppressor, or TVS diode, should be added from the supply rail to ground.

Detailed Schematic

Figure 1 shows the connection of the FSA4157. The drain of the measurement MOSFET, M1, is coupled via RVDS to pin 4 of the analog switch. When the MOSFET is on and current is being measured the path from pin 4 to pin 1 of the switch is closed, passing the signal. During the off-time the analog switch shorts the pin 4 input to ground to minimize the stray coupling of the rapidly rising large signal. Pin 1 should have loading but remember to consider the value of RVDS when calculating exact outputs.

The gate circuit of Figure 1 includes an extra enhancement with DZ1, that may or may not be necessary depending on your application. Because the GATE drive for the MOSFET reaches 10 volts, when the drive turns off the low threshold of the FSA4157 requires that drive to go below 0.8 volts to switch, by which time a large drain voltage spike can be observed at the CS output. The FSA4157 requires less than 5 volts gate drive (actually +2.4 for a high level). The 5-volt level shifting Zener in the gate line causes the GATE drive to the FSA4157 to drop nearly immediately, in any event prior to the MOSFET turning off for a clean GATE turn-off signal at CS.

Figure 1

An FSA4157-D analog switch used to implement low side RDS(on) sensing. Note that built in diodes clamp both the signal as well as the control voltage. During the interval that the MOSFET M1 is on, the signal passes from pin 4 to pin 1 of the FSA4157-D analog switch. When the MOSFET switches off, pin 1 of the analog switch is disconnected but the pin 4 input is grounded to minimize coupling of the switching pulse.

An FSA4157-D analog switch used to implement low side RDS(on) sensing. Note that built in diodes clamp both the signal as well as the control voltage. During the interval that the MOSFET M1 is on, the signal passes from pin 4 to pin 1 of the FSA4157-D analog switch. When the MOSFET switches off, pin 1 of the analog switch is disconnected but the pin 4 input is grounded to minimize coupling of the switching pulse.

Click here for larger image 

20kHz MOSFET switching with RDS(on) current sensing: 
CH1 (yellow): MOSFET gate drive
CH2 (blue): MOSFET drain
CH3 (violet): Analog Switch Output
CH4 (green): Analog Switch drive

CH3 Cursor 2 at 106mV indicates MOSFET RDS(on) drop during the interval when the MOSFET is on. This example is for a 3-amp load NTD24N06 rated at 0.032Ω RDS(on)

20kHz MOSFET switching with RDS(on) current sensing:

CH1 (yellow): MOSFET gate drive

CH2 (blue): MOSFET drain

CH3 (violet): Analog Switch Output

CH4 (green): Analog Switch drive

CH3 Cursor 2 at 106mV indicates MOSFET RDS(on) drop during the interval when the MOSFET is on. This example is for a 3-amp load NTD24N06 rated at 0.032Ω RDS(on)

Click here for larger image 

This trace shows the detail of how the analog switch drive is speeded up by the 5.1-volt Zener MOSFET gate driver so that the switch disconnects faster than the MOSFET switches off, minimizing coupling of transients through the switch.

This trace shows the detail of how the analog switch drive is speeded up by the 5.1-volt Zener MOSFET gate driver so that the switch disconnects faster than the MOSFET switches off, minimizing coupling of transients through the switch.

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