Electronic load

One common use for wirewound resistors seems to be as the load for a converter. I also use that configuration when doing thermals to simulate the customer s system and for noise and ripple measurements. But rarely do I use it for anything else. I would strongly suggest you get yourself a good electronic load. But do remember to set it to CC mode (constant current mode). Because a resistor (or an electronic load set to CR mode) is just too benign. For example, rarely does it reveal any fundamental start-up issues. 

Test your switcher s phase margins with something very close to the final load as early as possible. As mentioned previously, even simple startup problems of switchers do not usually show up with resistive loads. For these, you really need to test using an electronic load, placed at least in constant current (CC) mode. 

Question 2 Is my equipment somehow responsible That is sadly often the case. For example, some electronic loads can show weird glitches in the load profile they present to the converter under dynamic conditions. For example, if we are doing step load testing from 10mA to 200mA, all may be fine. But if we go from 0mA to 200mA, and see an output overshoot/undershoot, it could also be because of the electronic load. We may need to do... 

In general, most converters are tested on the bench with the electronic load set to constant current (CC mode). True, that s not benign, nor as malignant as it gets. But the implied expectation is that converters should at least work in CC mode. They should, in particular, have no startup issues with this type of load profile. But even that may not be the end of the story Some loads can also vary with time. For example, an incandescent bulb has a resistive profile, but its cold resistance is much lower than its hot resistance. That s why most bulbs fail towards the end of their natural lifetime just when you throw the wall switch to its ON position. And if the converter is powering a system board characterized by sudden variations in its instantaneous supply current demand, that can cause severe problems to the converter, too. The best known example of this is an AC-DC power supply inside a computer. The 12V rail goes to the hard disk, which can suddenly demand very high currents as it spins up, and then lapse back equally suddenly into a lower current mode. 

Question 9 Is there some interaction with nearby circuitry Yes, you could be picking up fields from nearby circuits, but that shouldn t affect a typical switcher, simply because it produces enough noise and fields of its own. However, it is a good idea to do the reverse-peel here. If I find the converter is on a larger system board, I immediately and carefully first cut off all the traces leading from its output and divert them to my predictable electronic load. I also cut the input traces and divert them to my bench power supply. If the problem is gone, it is an interaction problem. 

Data are still generally typed into a database rather than electronically loaded from other systems. The first step of any data entry process should involve a check for duplicate cases. The need for decision making at the data entry stage will depend upon the type of database design. In all cases, there should be clear rules on how data should be entered into each field to ensure consistency and aid subsequent searching and outputting. This is particularly important when there are multiple users distributed over a number of international sites. Use of electronically available field specific lists of value and well-defined coding conventions will help with this. 

The second alternative is an electronic load. This device is a circuit that has a controllable switch (typically a Darlington configured pair of bipolar transistors or a MOSFET) that can be modulated to conduct any level of current the user desires. An example of an electronic load circuit is presented in this chapter. The electronic load will be constructed piece by piece and tested separately. When all the pieces are constructed and simulated, the whole sum of the electronic load can be assembled and tested as a unit. 

The power section in this electronic load will be a N-channel power MOSFET. This design uses an IRF250, manufactured by International Rectifier . If a substitute part is used, pick a MOSFET that has the power, drain-to-source voltage, and current rating required for your range, and try to minimize drain-to-source on resistance (RDSon). The schematic of the power MOSFET and drive circuitry is shown in Fig. 5.1. 

Figure 5.1 Schematic of electronic load power stage.

One inconvenience associated with using operational amplifiers is the dual positive and negative power supplies that are frequently required. For convenience, and to limit the power source of our electronic load box to one power supply, we will include the following circuit in our electronic load. The circuit takes a 15 V DC signal and converts it to a -11 V DC signal. The schematic for this circuit is shown in Fig. 5.9. Note that the values shown are actual lab values of circuit. Standard resistor values are shown in parentheses. 

The final piece in the MOSFET electronic load puzzle is a method of adjusting the current limit. A simplistic yet flexible circuit to accomplish this is shown in Fig. 5.14. 

Figure 5.14 Schematic of electronic load reference and pulse load.

The breadboard pulse load waveform (from 300 mA to 5 A) is shown in Fig. 5.16. The IsSpice simulation results are shown in Fig. 5.17. Micro-Cap and PSpice results are shown in Figs. 5.18 and 5.19. In Figs. 5.15, 5.16, 5.17, and 5.19, the top waveform is the output of the comparator and the bottom waveform is the output of the analog switch (which would then be connected to the electronic load power circuit, such as that shown earlier in this chapter.)... 

Figure 5.16 Electronic load reference pulse output waveforms.

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