[MUSIC PLAYING] Now we are switching from all those flyback topologies to the forward converters. The forward converters, typically we use for the higher power range. The smallest one here is the single ended forward converter using only one switch. It's the simplest forward topology we have.
The major difference to the flyback is we're going to need two rectifiers. We're going to need two magnetics. So we have a non-gapped transformer, and we have a storage choke at the output. Of course, for all isolated topologies, we are able to adapt our windings ratios so we can adjust our duty cycle to best efficiency.
For the single ended forward, we need to know that the DC magnetic bias needs a core reset. So we have to add either auxiliary winding, or could be also active clamping. If the bias winding is similar to the primary, then duty cycle needs to be less than 50% for such standard single ended forward converters. Active clamping allows duty cycle up to 70%. So it's a bit better suited for wide range input.
Unfortunately, we are using the core only in one direction. We are using only a half cycle of the hysteresis, so it's a poor core utilization. The transformer will be big. We also have certain remaining ringing that could cause EMI in the RF range. We have pulsed current at the input. But we have continuous current at the output similar to a buck converter.
The voltage stress at the FET is two times the input voltage plus a certain amount of overshoot. The voltage stress at the rectifiers is related to input voltage and windings ratios, as we know already.
Synchronous rectification could be directly driven from the primary side either via auxiliary windings, or for small output voltage we can generate the gate drive directly out of the output windings. That's well-suited for active clamp typology here because we have clean wave forms that are well-suited to drive our synchronous rectifiers here.
Now I have a look on the primary side of the forward power stage. When we close the switch Q1, we are forcing a current across our primary winding. So this is causing a so-called DC bias but we need to reset our core. That's what we must keep in mind.
On the secondary side, for this non-gapped transformer the input voltage by windings ratio is present on our secondary side. So similar to a buck converter, we are now driving a forward current across our diode D1 to our storage choke and the output capacitor, delivering energy to the load and charging our output capacitor.
If we open now switch Q1 we are demagnetizing the core. So the voltage that is present at the demagnetizing bindings due to windings orientation is vice versa. And across input capacitor we are resetting here our core. That's important to know. So we are only using half of the hysteresis for the single ended forward converter.
On the secondary side, similar to a buck converter, the output voltage is present across our inductor L1. And that the freewheeling current is driven by the energy stored in the air gap across output capacitor and rectifier D2. [音乐播放] 现在，我们要从 所有这些反激式拓扑 切换到正向转换器。 一般来说，我们 将正向转换器 用于较高的功率范围。 此处最小的是 仅使用一个开关的 单端正向转换器。 它是我们所拥有的 最简单的正向拓扑。 与反激式的 主要区别是， 我们将需要 两个整流器。 我们将需要 两个磁性部件。 因此，我们有一个 无间隙变压器， 并且在输入端有一个 储能扼流圈。 当然，对于所有 隔离式拓扑来说， 我们能够 调整绕组比， 因此能够将占空比 调整到最佳效率。 对于单端正向， 我们需要知道， 直流磁性偏置需要 一次内核重置。 因此，我们必须添加 辅助绕组， 也可以是 有源钳位。 如果偏置绕组 类似于初级， 则对于此类标准 单端正向转换器， 占空比必须 小于 50%。 有源钳位允许 占空比高达 70%。 因此，它稍微更适合 宽范围输入。 遗憾的是，我们将仅在 一个方向使用内核。 我们将仅使用半个 周期的磁滞， 因此它是对内核的一种不良利用。 变压器将会很大。 我们还有一定的 残余振铃， 可能在射频范围内 导致 EMI。 我们在输入端 具有脉冲电流。 但我们在输出端 具有连续电流， 这类似于降压转换器。 FET 上的电压应力 为两倍的输入电压 加上一定量的 过冲。 我们早就知道， 整流器上的 电压应力 与输入电压 和绕组比有关。 可以从初级侧 通过辅助绕组 直接驱动同步整流， 对于小输出电压， 我们可以 直接从 输出绕组产生 栅极驱动。 这非常适合 此处的有源钳位 拓扑，因为我们具有 非常适合驱动 此处的同步整器的 干净波形。 现在看一下 正向功率级的 初级侧。 当我们闭合开关 Q1 时，将会强制电流 通过初级绕组。 因此，这会导致 所谓的直流偏置， 但我们需要重置内核。 这一点我们必须 牢记在心。 在次级侧，对于 此无间隙变压器， 次级侧存在 相当于输入电压 乘以绕组比的 电压。 与降压转换器 相似，我们 现在驱动正向 电流通过二极管 D1 到达储能扼流圈 和输出电容器， 为负载输送能量 并为输出电容器 充电。 如果我们现在断开天关 Q1， 则会对内核进行消磁。 因此，消磁绕组上 存在电压， 这是由于绕组方向 发生了逆转。 我们将在此处跨输入电容器 重置我们的内核。 知道这一点很重要。 因此，我们将 对单端正向 转换器仅使用 一半的磁滞。 在次级侧， 与降压转换器类似， 跨电感器 L1 存在 输出电压。 并且 续流电流 由气隙中存储的 能量驱动，通过输出 电容器和整流器 D2。 This website is under heavy load (queue full) We're sorry, too many people are accessing this website at the same time. We're working on this problem. Please try again later.
