Build your own oscilloscope probes for power measurements (part 2)
In Part 1, we covered the bandwidth limitations of passive probes and why they can't be used in today's switching power supply designs. High-frequency probes are expensive and often out of reach to many small companies. We also looked at the basic construction of a 50 Ω, 1:1 passive probe and discussed transmission-line effects that can distort signals and how to compensate for those errors. In this concluding part, we'll cover the design and construction of an n:1 voltage probe and a current probe.
50 Ω, n:1 voltage probe
Basic 1:1 passive probes are extremely useful so long as the signal reaching the oscilloscope isn't more than the input amplifiers can handle. While many commercial passive probes can operate in 1:1 and 10:1 modes, sometimes the 10:1 attenuation isn’t enough. For example, you can't connect AC mains voltages to a 10:1 probe without damaging your oscilloscope. You often need a 1000:1 probe. For other applications you may need a different attenuation ratio.
Requirements: Build a 50 Ω probe for a >1GHz oscilloscope to examine the power signals of a 110 VAC to 220 VAC, 300 W power-factor-correction circuit.
Maximum voltage: +400 VDC plus any spike voltages.
Devices under Test:
- D3 superjunction MOSFET, part number: D3S340N65B-U (VDSS=650 V, ID=12 A, RDS(on) = 360 mΩ(nom), tf >6.5 nS)
- CREE SiC Schottky Rectifier, part number: C3D04060A, (600 V, 7.5 A)
- Cut a length of RG174 (50 Ω) cable to approximately 5 in. (12.5 cm). Preferably, already having a BNC connector on one end.
- Strip 0.5 inches (12 mm) for the insulating Jacket from the cable. Unbraid the shield to the jacket. Strip 0.25 in (6 mm) from the center conductor insulation.
- Cut a length of 0.5 in (12 mm) of 0.5 in brass tubing and debur ends.
- The maximum voltage the probe will see is 400 VDC. Watch the VMAX rating of the capacitor. The desired display voltage is 100 mV/div (1000:1), which is 100 mV across the 50 Ω oscilloscope terminating resistor. The current (IS) through tip sense resistor is:
- Check the power dissipation in the sense resistor:
- Assemble the sense-end of the cable as in Figure 10.
The value of the sense resistor is:
Note: This power is within the power ratings of a single ¼ watt resistor, but it will get hot. I will use two 100 kΩ, ¼ watt resistors in parallel.
Figure 10. A 1,000:1 50 Ω voltage probe uses two 100 kΩ resistors in parallel, which doubles power dissipation over a single 50 kΩ resistor.
High frequency current probe
Common commercially available current probes have bandwidths from 60 MHz to 120 MHz. Viewing the high-frequency current waveform is important for estimating the switching losses within high-frequency semiconductor switches. Thus, you may need a higher bandwidth current probe. Figure 11 shows an example current probe.
Figure 11. A high-frequency current probe uses a toroid to capture fields from current in the wire.
The current probe in Figure 11, is essentially a 1:n forward-mode transformer with a high leakage ¼ turn primary. Because the primary is not an ideal one turn, its final accuracy will be addressed during its final calibration. The secondary turns are placed evenly around the toroid core.
You must place the terminating resistor immediately adjacent to the wound toroid. Doing so minimizes the transmission-line effects of the coax cable from becoming a significant portion of the oscilloscope input signal. The termination resistor prevents high signal currents from entering the cable. The oscilloscope termination should be set to 1 MΩ.
The impedance of the circuit branch through which the current is being measured, should be very low. The reflected impedance (insertion impedance) of the current probe should be kept as low as possible (low Rt) and still provide the desired amplitude for the oscilloscope input.
The current induced on the secondary winding is:
To convert this current into a voltage displayed on an oscilloscope, you must place a resistor across the secondary winding. This resistor can be any value, but the higher the resistance value, the more back EMF is placed on the primary’s target circuit. The back EMF manifests itself as an additional voltage drop in series with the target’s current path,which affects the primary's current flow and thus its accuracy. The amount of this error is proportional to the value of the resistance placed on the secondary.
The range of AC currents that are typically observed by the current probe can range from 100’s of amperes (1 kW supplies) down to milliamps (gate drive circuits). One current probe cannot address this range and still fall within the input dynamic range of the oscilloscope input. Hence several current probes are required, with differing turns ratios for the various current levels within a high frequency switching supply. The turns ratios are not cast in concrete, the common ratios are:
- 25:1 (10 A – 20 A)
- 50:1 (1 A – 10 A)
- 100:1 (0.5 A – 1 A)
Because the secondary current is small, the wire gauge need only be #32 AWG.
The terminating resistance can be estimated by:
The current probe shown in Figure 11 used a SMD resistor, which is why a small PCB was needed to mount it and to anchor the end of the coax cable. Be sure to conformally coat the entire current transformer assembly; the windings are very fragile and the coax cable can present a lot of mechanical stress to the current probe.