Two types of switching power supplies
High-Frequency Switching Power Supply (HF-SMPS)
HF-SMPS is a high-efficiency, highly robust power supply mode commonly used in electronic devices such as communication for LDO power supply of power management chips (PMICs), 1.8v logic circuits, RF circuits, and external loads. HF-SMPS does not support remote feedback, so output capacitors must be placed close to the chip pins. Since the HF-SMPS estimates the current by the voltage difference across the inductor, placing the capacitor remotely will increase the DCR of the feedback loop, thus causing errors.
Fast Transient Switching Power Supply (FT-SMPS)
FT-SMPS is a high precision, fast transient response power supply mode that supports powering application processors, GPUs, COREs, modems, etc. FT-SMPS supports multi-phase switching power supplies and supports remote differential feedback to detect dynamic load termination, so its output capacitors are placed close to the load side. The feedback modes of HF-SMPS and FT-SMPS are shown in Figure 1.
Operating Mode
PWM mode (Pulse width modulation)
When the circuit is operating at medium load or heavy load, the power supply will be in PWM mode when the inductor current is continuous. The switching frequency is optimized for transient response, ripple, efficiency, external devices, and other factors to obtain the best power supply performance. According to whether the inductor current is continuous, PWM mode can be divided into constant conduction mode (CCM) and discontinuous conduction mode (DCM), the voltage and current waveforms shown in Figure 2, Figure 3. In detail, DCM mode oscillation in the previous “BUCK power supply oscillation principle” article has been explained. Here it will not be introduced.
Pulse-skip PWM mode (Pulse-skip PWM)
When the circuit is working under light load, the efficiency in PWM mode is very low (MOS conduction, switching losses dominate), so the efficiency is improved by skipping some unnecessary pulses (reduce the number of switching). Because the skip pulse is still part of the PWM mode, it still keeps the output voltage ripple shallow. The waveform of the pulse-skipping PWM mode is shown in Figure 4, where the frequency of each pulse is constant, but the duty cycle gradually decreases.
PFM mode (Pulse frequency modulation)
PWM mode is a constant frequency pulse mode; HF-SMPS and FT-SMPS also support PFM mode, the pulse frequency variable. Because of the ability to reduce both the switching losses and the current losses on the ground, the PFM mode is more efficient at light loads. As shown in Figure 5, we observe that the PFM mode is similar to the jump pulse PWM mode described above, except that the pulse frequency is variable in the PFM mode. The output voltage ripple of PFM is the largest compared with the modes mentioned above.
Auto-mode
Auto-mode is a mechanism that automatically switches the circuit operating mode between PWM and PFM according to the changing load without software involvement. The PMICs used in today’s smartphones generally have this function. As shown in Figure 6, when the output current changes between 40mA and 400mA, PFM and PWM also change. Also, to prevent the output voltage from dropping in PFM mode, the controller automatically raises the output voltage of PFM; this balance is only available in HF-SMPS power supplies.
HC-PFM mode (High-current PFM)
HC-PFM mode is a brief PFM mode that occurs during the PFM to PWM conversion. When the circuit is operating in PFM mode, the load is suddenly aggravated, causing the output voltage to dip. Since it is impossible to switch to PWM mode quickly, some time is needed to warm up the PWM circuit. To prevent this warm-up process, the output voltage will be too low, controlling the PFM current between the normal PFM and PWM; this process of a few us is called HC-PFM, as shown in Figure 7.
Output voltage and ripple measurement
In the P0 backboard debugging of cell phone development, the test of power supply integrity is the top priority, which mainly includes the power-on timing of each power supply, the power supply voltage in various states of the phone, the heavy load power supply ripple, and the inductor current.
In the process of measurement, we need to pay attention to the following points:
- The voltage measurement must be selected on the load capacitance; the ground should be selected near the ground.
- The oscilloscope must be tuned to the 20MHz bandwidth limit.
- Try to use a short ground probe; the general supplier recommended using RF coaxial cable line to measure, but because of the cost and complexity. We generally measure and its sensitive power supply only when used, in addition to the use of ground springs is also a good choice.
