Testing

Turning outputs on/off shows a well behaved output voltage waveform with no tendency to overshoot or undershoot with various loads. The internal processor is monitoring key presses and controls the output appropriately. The output voltage can be adjusted in mV and the current limit in mA or 100 microamps steps for levels less than 1 A. A track option allows all the channels to track together in terms of their voltage and current settings. Each channel of the HMC8043 can be programmed to operate in constant voltage or constant current mode and has an impressive range of protection mechanisms including over-voltage, over-current and over-power protection. Electronic fuse values can also be defined for each channel along with a fuse delay from 10 ms to 10 s to avoid channel dropout at start up inrush. Fusing can be interlinked between channels. In addition to displaying voltage and current readings the instantaneous and accumulated power delivered to the load in W/s in can also be logged. This gives a useful indication of the power requirements and expected battery life particularly if the circuit switches between sleep and active modes.

The three isolated outputs allow the freedom to assign the available 100 W as you wish: connect all three in parallel to give 0-30 V at around 3 A or in series to give 0-99 V at around 1 A output. In this mode it makes sense to set the channels to track so that they share the load. When more than two outputs are connected in series it is possible to exceed the maximum 33 V reverse voltage allowable at the input terminals. This will occur with a load connected, when one of the channels in the series chain turns off due to a low current limiting setting. To ensure the input reverse voltage is not exceeded connect a maximum of two channels only in series.

The most important characteristic of a power supply is the reliability of the output voltage. Working on an expensive prototype you don’t want sudden load changes to cause the supply to fluctuate and damage components.

Turning outputs on/ off using the front panel buttons shows a well behaved output voltage level (Figure 1) with no tendency to overshoot or undershoot with various loads. Fan noise was only evident during periods of high power operation. Switched-mode supplies tend to be electrically noisy especially when operating at low load. The HMC8043 showed excellent performance, falling within the 4 mVpp ripple given in the specification.

A situation likely to catch out any microcontroller equipment is a hard turn-off caused by power outage or switch-off at the wall outlet. Unless the raw AC input is monitored closely the microcontroller doesn’t get enough warning and the control loop may go unstable as voltage drops. The Rohde & Schwarz HMC8043 reacts to this situation by superimposing an 8-ms (worst case) output spike of around 1.1 V on the output DC level before the voltage falls away (Figure 2). This feature may pose a problem for sensitive low voltage circuits; a 3.3-V or 1.8-V system would see its supply peak at around 4.4 V and 2.9 V respectively, and underlines the importance of including on-board regulation on any expensive prototype circuitry to protect against this type of event.