Pulse Width and Duty Cycle Limitations

Because the pulse current, width, and dead time are individually configurable for every pulse, it is not possible to describe every possible valid pulse sequence. To construct a pulse sequence with large pulse widths or high average current, be aware of these pulse width and duty cycle limitations. Use the provided figures and calculations to produce a valid pulse sequence.

Pulse Width Limitations

Higher pulse currents are limited to shorter pulse widths. Lower pulse currents and limited sequence lengths permit significantly higher pulse widths.

The following figure illustrates the maximum allowable pulse current as a function of pulse width. The maximum pulse width is 655.35 µs. For sufficiently low pulse currents, you can specify a dead time of zero (equivalent to a 100% duty cycle) after a given pulse to extend the pulse for the duration of the following pulse width. For two pulses unseparated by dead time, normal Pulse Parameter Limits still apply relative to the start time of the following pulse.

Figure 7. Maximum Pulse Current vs. Pulse Width


Note For a given pulse width, the maximum allowable pulse current is four times higher in the 10 A range than in the other ranges.

Duty Cycle Limitations

The PXIe-4133 limits the number and frequency of pulses to prevent overheating and to operate within the total power available for pulsing. The limits are a function of the pulse current, the pulse period (pulse width and dead time), and the number of pulses. The following table describes how duty cycle and duty cycle limits are calculated.

Table 4. Duty Cycle and Limit Calculations
Value Calculation for Uniform Pulses Calculation for Variable Pulses
Average Current Pulse Current × Duty Cycle (Sum of all (Pulse Current × Pulse Width)) / Sequence Length
Duty Cycle Pulse Width / Pulse Period
Pulse Period Pulse Width + Dead Time
Sequence Length Pulse Period × Number of Pulses Sum of all Pulse Periods

Duty Cycle Reference Curves (10 A Range)

The following figure contains four example reference curves (A, B, C, and D) that illustrate combinations of pulse current and pulse width.

Figure 8. Pulse Current vs. Pulse Width Reference Curves (10 A Range)


For each reference curve in the preceding figure, the following figure shows the permissible bounds for average current and pulse sequence length.

Figure 9. Maximum Pulse Sequence Length (10 A Range)


Note For pulse current and pulse width combinations below reference curve A, use reference curve A on the corresponding maximum pulse sequence length and average current figure. Reference curve A represents the maximum possible pulse sequence length and average current values for any combination of pulse current and pulse width below reference curve A.

The following example process demonstrates how to use the preceding figures to understand the relationship between pulse sequence parameters and validate a pulse sequence that consists of 26 pulses of 10 A with a 10 µs pulse width and 10% duty cycle.

  1. Identify the appropriate reference curve. For example, if you are supplying 10 A pulses with a 10 µs pulse width, use Reference Curve B.
  2. Assuming you are pulsing at a 10% duty cycle, you can determine the following values:
    • The average current is 1 A (10% of 10 A).
    • The pulse period is 100 µs (10 µs / 10%).
    • The dead time is 90 µs (100 µs – 10 µs).
  3. For pulses on reference curve B in a pulse sequence with 1 A average current, the Maximum Pulse Sequence Length figure indicates a maximum pulse sequence length of 2.6 ms. With a 100 µs pulse period, this allows for a maximum pulse count of 26 pulses.

Duty Cycle Reference Curves (0.1 A, 0.5 A, and 2.5 A Ranges)

The pulse sequence length and average current are more restricted in the 2.5A, 0.5A, and 0.1A ranges. The following figures illustrate the limitations in these ranges. The process for interpreting these figures is identical to the process described for the 10 A range, though specific values will differ.

Figure 10. Pulse Current vs. Pulse Width Reference Curves (0.5 A, 1 A, and 2.5 A Ranges)


Figure 11. Maximum Pulse Sequence Length (0.5 A, 1 A, and 2.5 A Ranges)


Software Validation for Pulse Sequences

The PXIe-4133 driver software validates every requested pulse sequence before executing the sequence. For information about validating a pulse sequence without generating pulses, refer to Validating a Pulse Sequence.

Unconditionally Valid Pulse Sequences

A pulse sequence that meets all of the following conditions will always be valid.

  • Every pulse width is ≤50 µs
  • The average current of every pulse is
    • < 0.34 A (for the 10 A range)
    • < 85 mA (for the 0.1 A, 0.5 A, and 2.5 A ranges)
In the 10 A range, this allows for up to 3.4% duty cycle with 10 A pulses, and 10% duty cycle with up to 3.4 A pulses.