In high-end telecommunication applications, you often face the challenges of delivering power across large-scale printed circuit boards (PCBs). To give precious real estate to critical ASICs and processors, the power supplies are often allocated to the corner or edge of the board. To compensate for the resistive drop of power path, remote sensing is often used – especially for low-voltage, high-current applications. The dynamic nature of the load, coupled with the parasitic resistance of the power path, may affect the operation of the power supply, if not attended. Below are 3 methods to avoiding pitfalls when using a remote supply:
Reduce power path impedance: By utilizing the available power planes, the DC-voltage gradient can be reduced to be within regulation tolerance. The power plane helps with DC regulation accuracy as well as improves system efficiency by reducing the resistive drop along the power path.
Split the output capacitance: It is important for a dynamic load, such as gate driver, to split the output capacitance between supply and remote load. The output capacitance at remote load acts as a bypass capacitor for the dynamic load. This reduces the ripple/noise current from the delivering path. It also stabilizes the output voltage at a remote sense point. This makes both the monitoring and sensing circuitry more accurate and reliable.
High-frequency bypass capacitor: Adding a high-frequency bypass capacitor at the local power supply is also beneficial. Modern converters are often equipped with a differential amplifier for remote sensing. Two sensing resistors are located near the remote side and connect the load voltage back to the controller by differential pairs. As shown in Figure 1, a synchronous buck controller like the TPS40400 has a differential amplifier which compensates the voltage drop on the parasitic resistor Rp due to the power path impedance.
Figure 1. Synchronous buck controller adjusted to output compensates the voltage drop on parasitic resistor Rp by dedicated differential amplifier DIFFO.
If there is no dedicated differential amplifier, you can still remote sense your power supply. A remote sense resistor connects the load voltage back to the converter. It compares to the reference voltage and regulates the output voltage. An example converter is given in Figure 2 where a step-down converter like the TPS62110 is able to remote sense the load and regulates the output for any parasitic resistor Rp voltage drop.
Figure 2: Step-down converter remote senses the load by a single resistor Rsns and regulates the output for any parasitic resistor Rp voltage drop.
However, when a dynamic load is applied as illustrated in Figure 2, the remote sense picks up the dynamic voltage and tries to compensate the voltage drop on the parasitic resistor Rp. This may result in a low-frequency oscillation due to the controller propagation delay. It appears as slight jittering on the switching waveform and causes elevated ripple on the output side. A high-frequency bypassing capacitor, Cbypass, can easily remedy the situation. It filters out the high frequency dynamic voltage while it keeps DC remote sense characteristics.
I tested a 7V gate-drive power supply with 1 uF bypass capacitor using the TPS62110, a step-down converter. It clearly removes a 33 kHz oscillation from the output voltage and results in a low-output ripple, 20 mV, 0.3% of the regulated voltage. Figure 3 shows the original output ripple with 33 kHz oscillation, while Figure 4 shows low-output ripple without oscillation. The oscillation is removed by 1 uF bypass capacitor.
Figure 3: Original output ripple showing 33 kHz oscillation
Figure 4: A 1 uF bypass cap removes the oscillation with low output ripple
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