Fast edges, high current: current probes for SiC and GaN
Why an oscilloscope current probe needs both higher bandwidth and a higher usable frequency to keep up with modern power semiconductors, and how the new HIOKI CT6704 and CT6705 deliver them.
Power semiconductors have moved on. SiC and GaN switch faster and at higher frequencies than the silicon they replace, and that puts two very different demands on the current probe you watch them with.
You need the bandwidth to actually see the switching edge instead of a rounded-off approximation of it. And you need the probe to still carry a useful current at the frequency where that switching happens, rather than forcing you onto a bigger probe just to survive it.
The CT6704 and CT6705 were built around exactly these two demands.
Fast enough for steep edges
Switching transitions in SiC and GaN are so fast that a probe only reproduces them faithfully if its bandwidth reaches well beyond the fundamental.
The CT6704 replaces the 3274, the CT6705 replaces the 3275, and the step in bandwidth is large.
- The CT6704 covers DC to 30 MHz with an 11.6 ns rise time, where the 3274 reached DC to 10 MHz at 35 ns.
- The CT6705 covers DC to 15 MHz with a 23.3 ns rise time, against the 3275's DC to 2 MHz and 175 ns. That works out to roughly three times the bandwidth on the one and more than seven times on the other, with correspondingly shorter rise times.
On a real signal, that is the difference between seeing the edge and seeing an approximation of it.
Rated current, well into the kHz
A current probe also has a second limit, and it's the one that quietly pushed people toward a probe with a higher current rating, and usually a lower bandwidth to go with it: derating. As the frequency rises, a probe can no longer carry its full rated current, because it would overheat. The allowed value falls the higher the frequency climbs. The reason is heat, specifically the eddy currents that high-frequency currents stir up in the metal inside the probe.
With the older probes, that ceiling sat surprisingly low. The 3274 held its full rated current only up to a little over 100 Hz before the curve began to fall; the 3275 made it to 400 Hz. For modern, fast-switching power electronics that is far too low: switching frequencies now sit well above it, in a range where the old probe could only carry a fraction of its nameplate current. The usual fix was a probe with a higher current rating, which usually came with lower bandwidth and so a slower rise time. You bought current and paid in speed.
The CT6704 and CT6705 address this through a magnetic and thermal design that holds internal heating down at high frequency. At room temperature the CT6704 holds its full rated current all the way to 100 kHz and the CT6705 to 10 kHz, where the predecessors ran out at a little over 100 Hz and 400 Hz respectively.
The speed stays, the drift goes
The 3274 and 3275 read the DC and low-frequency part of the current with a Hall element.
A Hall element is a semiconductor, and its balance shifts with temperature: when the probe gets warmer, the baseline drifts.
- And a probe gets warm easily: In a warm environment,
- say close to the power electronics as they heat up over time
- and above all through high-frequency signal content that warms it from the inside.
The current you are measuring is not the cause, though: direct current barely heats the probe.
The CT6704 and CT6705 replace that Hall element with a fluxgate, without giving up the high bandwidth these probes depend on. If anything, the bandwidth went up.
And the fluxgate brings its real strength with it:
- a baseline that barely drifts with temperature, around 90 % less DC drift than before
- for the first time on a probe for oscilloscopes an offset temperature coefficient on the datasheet (±0.1 mV/°C)
- and an operating temperature of −10 to +50 °C instead of 0 to 40 °C
It is the same sensing principle behind the precision sensors that pair with our power analyzers, a proven detector now arriving in a clamp-on probe with a standard BNC plug. (If you've ever wondered where the slightly odd name "zero-flux" comes from, we told that story separately, with a small detour through Back to the Future.)
