|Figure 1: A simplified schematic of|
a switch-mode power supply circuit
What does a basic switch-mode power supply circuit look like? In Figure 1, the input side on the left begins with a diode and capacitor that serve to rectify the AC input. That's followed by a switching transistor, typically a MOSFET, driven by a pulse-width-modulation (PWM) controller that determines how much DC power is seen on the far side of the transformer. Some of this signal is fed to an op amp that provides feedback to the PWM modulator so it can properly adjust the DC voltage depending on the load.
A number of measurements are possible on both sides of the transformer. On the input side, which is not referenced to ground, measurements include:
- Inrush current
- Power quality
- Line power
- Switching loss
- Safe operating area
- Control loop
- Dynamic on-resistance
On the ground-referenced output side, the roster of measurements include:
- Step load response
- Transformer B-H (magnetization)
- Turnoff characteristics
Voltages on switch-mode power supplies present some measurement challenges. They cover a wide dynamic range from millivolts to kilovolts. On the input side, they're not referenced to ground. You have to be concerned with effects of circuit loading. And, it's an inherently noisy environment; that noise can couple into your probe and/or the signal you're trying to measure.
|Figure 2: The "complete" voltage circuit comprising a|
non-ground-referenced DUT and oscilloscope
The circulating current in the ground loop creates a voltage across any impedances within the loop. For AC components, the predominating loss element is inductive. The ground lead at the tip of the probe forms an inductor across which the AC components of this voltage appear. Thus, the voltage at the probe input of the oscilloscope is not equal to the voltage at the probe's tip. Oscilloscopes measure the voltage at the BNC connector, not the probe tips.
|Figure 3: A differential amplifier solves the problem of|
measuring non-ground-referenced voltages
The differential amplifier takes in the two input signals and subtracts them from each other, which eliminates the need for a ground connection. It also has the benefit of canceling out any common signal on both of the probes (common-mode rejection). Thus, noise coupled from the MOSFET's switching will typically get into both probes and be canceled out. An example of a popular differential amplifier for these kinds of applications is Teledyne LeCroy's DA1855A.
Next time, we'll begin exploring some of the ways in which you can reduce measurement errors when digging around in a switch-mode power supply circuit.