Improving RFPA System Efficiency Using DC/DC Converters

From a power budget perspective, RF power amplifiers (RF PAs) powered directly from batteries are important components to consider. Traditionally, RF PAs used in CDMA/WCDMA cellular standards are powered directly from batteries, which makes system design easy, but the actual efficiency of linear power amplifiers used in these standards is very low over the entire transmit power range. This article describes a solution to provide efficient RF PA system power management through DC/DC converters.

As cellular standards continue to evolve, transmission rates have evolved from 14.4kbps in the CDMA-1 standard to 2Mbps in the CDMA2000/WCDMA standard. In addition, in order to increase the average revenue per user, cellular communication operators have begun to add services related to 3G phones. At the same time, talk time and battery life are also expected to be improved using batteries with the same or slightly higher capacity. This makes system design more challenging. System designers must be very careful and examine the power of each component on the mobile phone circuit board. From a power budget perspective, RF power amplifiers (RF PAs) powered directly from batteries are important components to consider.

The modulation circuit used in CDMA and WCDMA results in an AM signal that exhibits a non-constant amplitude envelope. To maintain signal integrity and promote spectrum regeneration, a linear power amplifier is required. However, the conversion efficiency is not high because the power amplifier can only maintain high efficiency when operating under gain compression conditions. In order to achieve the required linearity, the actual transmission power starts to compensate from the compression point of the power amplifier, which leads to an overall reduction in efficiency. When the mobile phone is operating in transmit mode, the power consumption of the RF part will account for 65% of the total power budget due to the actual low efficiency of the power amplifier.


Figure 1: Old and new methods

Therefore, using a magnetic buck converter to power the linear power amplifier is an ideal choice, which will greatly improve the efficiency of the system. Increased power efficiency (PAE) is the main performance indicator of the power amplifier.

PAE(%)= (POUT-PIN)/PdcThe

main purpose of using a DC-DC converter (power amplifier power regulator) is to reduce the Pdc factor in the denominator. When the power amplifier is directly connected to the battery, Pdc=Vbatt*Ibatt, while when it is powered by the power amplifier power regulator, Pdc=V o*I o. We can now see that to increase PAE, Vo and Io must be lower than Vbatt and Ibatt. This can be achieved by reducing the transmitted RF power level, which reduces the output voltage of the PA power regulator. This also reduces Io (the current drawn by the PA), and due to the high efficiency of the DC-DC converter, the battery output current is reduced.

To truly understand the power savings that the power regulator brings to the PA, it is important to consider the power probability plots for different modulation methods (see Figure 2). The power probability plots will be different in urban and rural areas.



Figure 2: In a standard mobile phone, the PA transmits at low power levels most of the time, and using a PA power regulator increases the probability of power savings.

As shown in Figure 3, in order to meet the adjacent channel power rejection ratio (ACPR) requirements, the output voltage of the DC-DC converter must vary with the transmitted power level. In the power level range of 0d Bm to 20d Bm, the battery current can be saved by up to 50mA. Figure 2 shows the PA operating at this power level range most of the time.


Figure 3: Battery current savings when the PA is powered from a DC-DC converter



Figure 4: Percentage power savings when the PA is powered from a power regulator

So why do we have to change the DC-DC converter voltage when the transmit power level increases? The answer is: this change is necessary to maintain the adjacent channel power rejection ratio (ACPR). ACPR is a measure of the tendency of a PA's distortion and other subsystems or systems to cause adjacent channel interference. It is the ratio of the main channel's power spectral density (PSD) to the power spectral density measured at several different offset frequencies.

Figure 5: How the PA's supply voltage and POUT affect ACLR