DC1000A 25A DC Current Bias supply - How it Works
In most power electronics circuits, wound components such as inductors, chokes and transformers must function properly in the presence of high levels of DC current.
If an inductive component is faulty then the whole power supply or inverter will not work properly and will have to be reworked or scrapped.
To guarantee the quality and performance of such a component it must be tested at real working conditions, including DC current bias, during manufacture.
The revolutionary design of the DC1000A eliminates the need for bulky and inflexible DC Bias Supplies that are tied to a particular LCR meter and restrict test performance.
Fig 1 Basic circuit showing measuring of inductance in the presence of a DC bias current
1 Basic Theory
When measuring inductance in the presence of DC Current Bias, the DC Bias supply is connected in parallel with the inductor under test and the LCR meter.
DC bias current, IDC flows in the left-hand side of the circuit and AC test current from the LCR meter flow in the right-hand side of the circuit.
The LCR meter AC current (I AC), and the AC test voltage are used to provide the desired inductance measurement.
A conventional DC Current Bias supply is built using series transistors to regulate the DC current and isolate the LCR meter from the low impedance DC supply.
The collector current of the transistor (Ic) is the DC current (IDC) supplied to the inductor under test.
Any AC test current that strays into the DC circuit on the left-hand side is an error current and will create inaccuracies in the AC measurement of inductance
2 Causes of Error
There are multiple paths for AC error current as shown below:
Fig 2A Circuit with paths for AC error current
There are three main sources of error (dIAC)
a) dIAC1
The Collector-Emitter capacitance of the regulating transistors.
Changes in the collector-emitter voltage of the transistor due to the LCR meter's test signal create changes in the transistors' capacitance
Fig 2B Transistor ideal and real characteristics
b) dIAC2
AC Variation in the DC current because of the imperfect load line of the transistor.
In an ideal transistor, the relationship between collector current and collector emitter voltage is flat above the activation point In practice all transistors show a slope of the shown “real” characteristic.
Even with constant base current, the collector current will tend to change with changing collector-emitter voltage. (see fig 2B).
As a result, the AC voltage from the LCR meter will cause error currents to flow in the transistors.
The magnitude of this error current will change for different settings of DC bias current.
This is because the slope of the load line increases at higher DC currents.
(Compare the magnitude of delta IC between Ib = 10mA, 5mA & 3 mA below)
This makes it very difficult to accurately model this effect and hence remove the error.
c) dIAC3 Capacitance between the DC signal path and chassis ground.
This inherent capacitance in the DC source, will cause a reduction in the current seen by the LCR current measure circuit, and hence produce and error in the LCR measurement.
Compensating accurately for all three of these errors at all combinations of test impedance, DC current and LCR meter signal level is very difficult if not impossible.
Until now, DC Bias supplies were restricted to a particular manufacturer's LCR model number in an attempt to minimize these errors,
3 The Solution
With the DC1000A's circuit, these errors are essentially eliminated by the use of an inductor placed inside an AC control loop.
A sense winding on the inductor produces a voltage proportional to the AC voltage across it.
A high gain control loop (shown greatly simplified here) injects an AC signal across the inductor so as to drive the AC voltage across the inductor towards zero.
This circuit responds with high gain (up to 10,000x) and and high frequencies (10 MHz) so that the AC input impedance of the DC Bias unit, as seen by the LCR meter, is very high.
A similar circuit is used in the DC bias supply return path to eliminate errors due to the capacitance to chassis ground.
The typical error current for this circuit at 100 kHz is less than 100uA, compared to 10's of mA using conventional methods.
Fig 3 improved method using AC correction method on both high and low lines.
4 Advantages
The user advantages of the circuit are many and significant:
1. The AC error current drawn by the DC bias supply is reduced by a factor of 100 (typical). This instantly produces a more accurate result from the LCR meter.
2. Any residual error is constant and independent of the DC bias current, so can easily be compensated out by traditional LCR methods.
3. Now open and short compensation can be applied by the LCR meter as usual to make measurements free from all stray effects.
4. The DC1000A requires no special connection to the LCR meter, or knowledge of its settings, making it suitable for use with any LCR meter.
5. Multiple DC1000's can be easily connected in parallel for testing with bias up to 500A (20x DC1000).
6. The DC1000A is smaller and lighter than any other high current DC Current Bias Supply.
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