|
|
|
|
Usage of Resistors |
|
|
 |
Current Detecting Resistors |
|
The figure below shows the current detecting
circuit of a DC/DC converter. The voltage across the Current Detecting Resistor
is fed back to control the output power. The resistance should be low to reduce
power dissipation, and the resistor should stand against repeated rush current.
Furthermore the self-inductance should be low for high frequency applications.
Refer to "Sensing a current
of DC/DC converter".
Recommended types are general purpose flat chip resistors SR73,
flame
retardant plastic mold resistors SL TSL
NPR. As for high frequency DC/DC converters,
metal-plate chip resistors TLR best
fit in. |
|
|
|
Resistors with reduced self-inductance improves
sensing accuracy |
|
A typical application of the resistors of
extremely low resistance is to detect the current in switching regulator circuit.
But if there exists a parasitic inductance, then it may cause an error to the
measurement. Assuming that di/dt=30A/us, R=1m , then self inductance
of the resistor =1nH. ( A typical through hole has an estimated 0.5nH of inductance.)
Then, as large as 30mV of spike voltage is correspond to 30A. Resistors are
produced to minimize the self-inductance, but trimming is a factor to increase
it. It is better to select reduced inductance resistors such as LR72 and
TLR for high frequency applications. |
|
 |
|
|
Practical method to measure the electric current(Four
point probe method) |
|
It is convenient to add a resistor to the
network to measure the electric current. Measuring the voltage across the resistor
gives the current that flows through it. The resistance should be low enough
not to disturb the circuit condition and dissipate spurious power. But when
the resistance is very low, the resistance of the printed circuit pattern may
cause an error to the measurement. A precision resistor has no longer any worth
in that case. A practical circuit patterning solution for it is known as Kelvin's
four point probe method, where the terminals to supply current and the terminals
to detect voltage are prepared as illustrated. Then the detected voltage is
exactly proportional to the current, regardless the resistance of current supply
printed pattern, how large it may be. (Input resistance of the voltmeter (not
shown) is assumed larger enough than the current detecting resistor, of course.)
There are special purpose resistors CSR
with built in four terminals, for precise electric current measurement. |
|
 |
| |
Diagram of CSR-structure |
|
 |
|
|
|
Resistors to detect the over-current |
|
As a means to detect the current passing through
the transistor, a resistor in series is added between an emitter and a ground.
This resistor should neither emit smoke nor catch fire even when the switching
transistor breaks down to be subjected to a larger current. In addition, reduced
parasitic inductance is required, particularly for the high frequency switching
control. Recommended resistors with low resistance are metal-plate type BPR SL
TSL LR72  ,
and/or LR. |
|
|
Application Notes
Brush up your circuit
Basic Knowledge
