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Application Note 1192
Efficiency
With a maximum available average input power of 12.95W,
efficiency becomes a key parameter to consider. At these
low power levels, even a few tens of milliwatts of power loss
would lower the efficiency. The following is a summary of the
approximate losses:
Power Transformer - 900mW
Primary FET - 100mW
Secondary Switch (FET in ISL6844EVAL1Z) - 100mW
Current Sense Resistor - 200mW
Fixed Control (Bias, Gate Drive) - 300mW
giving a total loss of about 1.6W (or 12.3%). This allows the
solution to hit the 87% efficiency target.
In the case of a diode-rectified circuit (ISL6844EVAL2Z), the
VI loss across the diode would be about 1W, resulting in an
additional 7% power consumption. The advantage of this
version is the cost savings achieved by not having to use the
relatively expensive gate drive circuitry.
ISL6844EVAL2Z Performance and
Differences
The major differences in performance of the conventionally
rectified design versus the synchronous rectified solution
have been highlighted in earlier paragraphs. The
performance variations are shown below:
Figure 7 shows the efficiencies of the ISL6844EVAL2Z board
with and without an additional pre-load resistor. The
additional load is necessary to ensure regulation even at
light loads (0.35W). The board can be used without the pre-
load resistor if the minimum load seen by the system is
greater than 0.2A.
90
85
80
FIGURE 8. PRIMARY GATE DRIVE - Vgs of Q2
CONDITION: V IN = 48V, I OUT = 3.1A
The primary FET gate waveform is shown in Figure 8. Since
cross-conduction between the primary and secondary is not
an issue here, a small gate resistor was sufficient. Adding a
gate resistor helps in reducing the turn-on spikes, as well as
damping oscillations of the parasitic tank circuit between the
transformer ’s leakage inductance and the FET capacitance.
Load regulation is shown in Figure 9. As discussed under
“Rectification”, the regulation is loose as compared to the
synchronous rectified version. However, in most
applications, very tight regulation is not a requirement.
115
114
113
112
111
110
109
108
107
106
105
104
75
70
65
WITHOUT PRE-LOAD RESISTOR
WITH PRE-LOAD RESISTOR
103
102
101
100
99
98
97
Vin = 48V
60
96
95
0
0.5
1.0
1.5 2.0 2.5 3.0
3.5
55
OUTPUT CURRENT (A)
LOAD REGULATION WITH PRE-LOAD RESISTOR
50 0
0.5
1
1.5 2 2.5
OUTPUT CURRENT (A)
3
3.5
FIGURE 9. LOAD REGULATION
FIGURE 7. TYPICAL EFFICIENCY CONDITION: V IN = 48V
5
AN1192.3
March 12, 2007
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ISL6844EVAL1Z 功能描述:EVALUATION BOARD 1 ISL6844 RoHS:是 类别:编程器，开发系统 >> 评估板 - DC/DC 与 AC/DC（离线）SMPS 系列:- 产品培训模块:Obsolescence Mitigation Program 标准包装:1 系列:True Shutdown™ 主要目的:DC/DC，步升 输出及类型:1，非隔离 功率 - 输出:- 输出电压:- 电流 - 输出:1A 输入电压:2.5 V ~ 5.5 V 稳压器拓扑结构:升压 频率 - 开关:3MHz 板类型:完全填充 已供物品:板 已用 IC / 零件:MAX8969
ISL6844EVAL2 功能描述:EVALUATION BOARD 2 ISL6844 RoHS:否 类别:编程器，开发系统 >> 评估板 - DC/DC 与 AC/DC（离线）SMPS 系列:- 产品培训模块:Obsolescence Mitigation Program 标准包装:1 系列:True Shutdown™ 主要目的:DC/DC，步升 输出及类型:1，非隔离 功率 - 输出:- 输出电压:- 电流 - 输出:1A 输入电压:2.5 V ~ 5.5 V 稳压器拓扑结构:升压 频率 - 开关:3MHz 板类型:完全填充 已供物品:板 已用 IC / 零件:MAX8969
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ISL6844IB 功能描述:电流型 PWM 控制器 Single-End Current RoHS:否 制造商:Texas Instruments 开关频率:27 KHz 上升时间: 下降时间: 工作电源电压:6 V to 15 V 工作电源电流:1.5 mA 输出端数量:1 最大工作温度:+ 105 C 安装风格:SMD/SMT 封装 / 箱体:TSSOP-14
ISL6844IB-T 功能描述:IC REG CTRLR BST FLYBK ISO 8SOIC RoHS:否 类别:集成电路 (IC) >> PMIC - 稳压器 - DC DC 切换控制器 系列:- 标准包装:2,500 系列:- PWM 型:电流模式 输出数:1 频率 - 最大:500kHz 占空比:100% 电源电压:8.2 V ~ 30 V 降压:无 升压:无 回扫:是 反相:无 倍增器:无 除法器:无 Cuk:无 隔离:是 工作温度:0°C ~ 70°C 封装/外壳:8-DIP（0.300"，7.62mm） 包装:管件 产品目录页面:1316 (CN2011-ZH PDF)
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ISL6844IRZ 制造商:Rochester Electronics LLC 功能描述: 制造商:Intersil Corporation 功能描述: