Transient Response Test
From: Author: Publish time:2011-09-24 13:58 Clicks:629
Transient Response Test
The transient response test is a dynamic test, providing the stable output for the measured power supply periodically switching between two programmed load conditions.
In response to the current jump of the electronic load, the CV power supply will output ring voltage which causes the power supply feedback loop in disorder and forms voltage overshoot and voltage drop, thereby affecting the equipment operation reliability, and even damaging the voltage sensitive components. The transient response test can not only assess this performance from application aspect, but also reveal the critical defects causing the instability from the production aspect, such as the output capacitance’s ESR, ESL, capacity, the feedback loop response time, phase margin and the system maximum transient output current, etc.
1.2.1. Transient Response Process Analysis
The principle of voltage overshoot is nearly the same with that of voltage drop. Here, we only discuss how current rise causes ring voltage, just as the above diagram shows:
(a) When the current rises according to the programmed current rising slew rate, due to the output circuit inductance and the influence of capacitance ESL, the output voltage, together with current rising waveform, will drop to VESL and forms the first wave trough of the voltage waveform.
(b) When the current becomes stable, due to the DC impedance of the output circuit and capacitance ESR, the voltage will recover to VESR and forms the first step.
RESR≈ (Va- VESR) /(Ib-Ia)
VESL ≈ Va –(LESL*Rate + (Ib-Ia)* RESR)
LESL ≈ (VESR -VESL) /Rate
(c) During this period, the power supply feedback loop is in disorder state. The loading current of the electronic load starts continuous discharge to the output capacitance(capacitance: COUT)until the feedback loop is established. Then the second valley value (VC) of the voltage waveform is formed. The discharge time (TD) is the feedback loop response time.
COUT ≈ TD*Ib / (Va - VC)
VC ≈ Va - Ib * TD / COUT
(d) When the voltage starts to rise from the second valley, the power supply starts charging to the output capacitance at the maximum current (ISYS) until the first peak valley Vd is formed. The charging duration is TC.
(VESR - VC) *TD/Ib ≈ (Vd - VC) *TC/( ISYS -Ib)
ISYS ≈ Ib * [(Vd - VC) / (VESR - VC) * TC/TD + 1]
(e) Thereafter, the voltage waveform will be in damping oscillation state. The smaller the oscillation cycle number n is, the greater the phase margin of the power supply loop is and the more superior the quality is.
1.2.2. Basic Requirement of Transient Response Test
a) Full range current rising time
According to the formula VESL ≈ Va –(LESL*Rate + (Ib-Ia)* RESR), the bigger the current change is and the higher the current rising rate is, the more obvious the change of the voltage overshoot and drop is and the more accurate the measurement is. In addition, the charge and discharge process of the output capacitance is interwoven with the response of the inductive reactance to current changes. In order to minimize the cross impact, it requires that the current transition time should be as short as possible, at least should be 5 times faster than that the feedback loop response time of the measured power supply. So transient response test has a very high requirement for the speed of the electronic load. The speed of the electronic load is generally expressed by the full range current rising time. The full range current rising time of the JT63 series electronic load is 10uS. Some people also use upper limit of the current rising time to describe the electronic load speed. Sine the upper limit of the current rising time=rated current/ full range current rising time, this index is not comparable among those equipments with different rated current.
b) Current slew rate programming
It is just because that the current changes has a big influence on the transient test, in order to make the measurement more objective, the test must be made at the same current rising slew rate. Thus, it is essential to have the function of current slew rate programming.
c) Electronic load D/A adjusting frequency
In respond to the current change, the inductive devices require the current to be steady and change continuously. While the current changes of the electronic load is realized through D/A digital processing, so D/A adjusting frequency directly determines the smooth degree of the current waveform and affects the measurement accuracy. The D/A adjusting frequency JT631 series electronic load is 500KHz.
d) Electronic load transient data measurement
JT63 series electronic load supports transient data analysis and measurement and real-display of the peak voltage (Vp+) and the valley voltage (Vp-). When the current rising slew rate is set very high or when ESL is very big and the influence quantity of ESL is more than that of the output capacitance charging and discharging, ESL will cause the first peak voltage, Vp+ and the first valley voltage(VESL),Vp-. Otherwise, if current rising slew rate is set very low or output capacitance very small, the influence quantity of the output capacitance charging and discharging is more than that of ESL, the output capacitance discharging will cause the second peak voltage Vp+ and second valley voltage (Vc) Vp-. If it is the first case, the Vp+ & Vp- measured value can directly show if there is some problems to the ESL and ESR of the output capacitance; if it is the latter case, Vp+ & Vp- measured value can directly show if there is some problems to the capacity value of the output capacitance. Users can also use the highest current slew rate to check the quality of the ESL and ESR first and then user a lower current slew rate to detect the capacitance capacity value and feedback loop response time.
e. Additional notes
Making the electronic load current as close as possible to the rated current of the power supply can significantly improve the measurement accuracy. Also note that sweep frequency in the transient test should not be too high so as to make the capacitance charging and discharging process reach steady state and improve the test accuracy. JT63 series electronic load provides dynamic frequency sweep function, which can help users capture the sweep frequency in the worst conditions, and meanwhile capture the peak voltage and valley voltage of this sweep frequency??. The transient response test, the connecting line not only exists impedance, but also has a bigger inductive reactance, and the influence is very significant. In order to avoid such influence, it is recommended to use the remote sensing function of the electronic load to make the test.