Sequence Your Power-Supply Outputs To Ensure Performance

May 12, 2011
Lab power supplies provide a number of ways in which outputs can be sequenced so as to power up a DUT in proper order. Agilent's Bob Zollo takes you through the options.

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Fig 1. Many devices, such as this PC motherboard, often require specific control over the timing and sequencing of their bias voltages.

Fig 2. The Agilent N6700 (left) for ATE systems offers hardware sequencing built into the power supply. The Agilent N6705B (right) provides sequencing capability for benchtop applications.

Many devices require multiple power-supply bias inputs to drive them. Commonly, we think of a circuit board requiring +5 V and ±15 V, but there are many other devices and power-supply configurations. From discreet transistors to ICs to circuit boards to final products, there often is a need to control the timing and sequencing of bias voltages (Fig. 1). Failure to follow the correct sequence could cause improper operation, or excessive current flow caused by latch-up, or even catastrophic failure of the device under test (DUT).

Output sequencing is all about controlling the timing of when the power supplies come on. But for most power supplies, the time it takes a power-supply output to turn on isn’t specified. The time it takes the power supply to transition from one voltage to another, typically called the power-supply programming response time, may be specified, but that’s only part of the total picture.

If you’re controlling the power supplies manually, then to control the sequencing, you need to know the time from when you press the “on” button to when the power supply reaches its programmed output voltage. If you’re controlling the power supply via a remote interface (such as GPIB, LAN, or USB), you need to know the time from when you send the “on” command to when the power supply reaches its programmed output voltage. Often, these turn-on times aren’t specified, leaving you to try to characterize the power supply’s behavior and then hope it’s repeatable.

Manual Power-Supply Sequencing

Manual sequencing is quite straightforward. You simply press the “on” button of each power supply in the order that the bias supplies need to be applied. Manual sequencing is only suitable for applications where the power-on order matters, but timing isn’t critical. Certainly, with a person pressing buttons, you cannot expect to achieve tight or repeatable timing.

The uncertainty of power-supply turn-on times probably isn’t important here, as the human factor of pressing buttons takes the most time. With manual sequencing, the best you can achieve is guaranteeing that output 1 comes on before output 2, which comes on before output 3, etc.

Computer-Controlled Sequencing

Better timing control can be achieved using a computer to program the power-supply outputs. When using a computer, you can improve timing accuracy by first sending the “on” command to get the power supply into its “on” state, which can take much longer than simply changing from one voltage to another.

Send this “on” command during a part of the program where timing is not critical. Then, when you need to sequence the outputs, you can send the command to change from 0 V to the appropriate bias voltage. The programming response time of the power supply (i.e., the time it takes to go from one voltage setting to another) is normally fairly repeatable and may even be specified, so it will be possible to account for it when you’re creating the program with proper timing. Note that the programming response time on some power supplies can be hundreds of milliseconds, so this will limit how fast the sequence can execute.

The key issue with computer-controlled timing is the jitter in the computer’s operating system. To control sequence timing, you will need to create a software timing loop. Even a carefully developed program will have some jitter as it executes, perhaps even as much as 10 ms or more. This jitter will lead to variability in timing as to when the voltage programming commands are sent to each power supply and thus lead to an output timing sequence that is not repeatable.

This computer-controlled method is suitable for DUTs where timing must be controlled to within 100 ms or higher.

Custom Hardware Sequencing

If more precise and repeatable control is needed, you may have to turn to custom hardware. I have seen systems where the engineers have built custom circuitry that sits between the power-supply outputs and the DUT. The custom circuitry is effectively a sequenced hardware switch that applies the power-supply output voltage (which is already programmed to the right value) at the precise time needed in the sequence.

This is a very costly and complex way to generate a sequence, but it can be very precise. Note that as current levels go up, designing a switching system can become very complex and expensive. For most test engineers, creating custom hardware just for sequencing isn’t a viable option, but for those who must have precise sequencing, it may be the only option.

Power Supplies With Built-In Sequencing

A few power supplies on the market today are specifically geared to sophisticated test (Fig. 2). These power supplies have built-in output sequencing. Rather than relying on a computer program to control the timing of the turn-on of the outputs, these supplies use internal hardware timers that allow the power supply to be programmed to turn on at specific intervals.

This eliminates the jitter found in software timing loops and provides hardware timing, accuracy, and repeatability. To create a multiple power-supply turn-on sequence, multiple supplies are linked together through a trigger signal or similar synchronization signaling method. The products are primarily available for automatic test equipment (ATE) systems, but some bench power systems provide built-in sequencing as well.


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